Uniformize usage() and extra_usage() message ending for commands helper. (#8268)

# Description

Working on uniformizing the ending messages regarding methods usage()
and extra_usage(). This is related to the issue
https://github.com/nushell/nushell/issues/5066 after discussing it with
@jntrnr

# User-Facing Changes

None.

# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/src/math/abs.rs

@@ -18,7 +18,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Returns the absolute value of a number"
+        "Returns the absolute value of a number."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/math/avg.rs

@@ -19,7 +19,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Returns the average of a list of numbers"
+        "Returns the average of a list of numbers."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/math/ceil.rs

@@ -18,7 +18,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Returns the ceil of a number (smallest integer greater than or equal to that number)"
+        "Returns the ceil of a number (smallest integer greater than or equal to that number)."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/math/floor.rs

@@ -18,7 +18,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Returns the floor of a number (largest integer less than or equal to that number)"
+        "Returns the floor of a number (largest integer less than or equal to that number)."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/math/ln.rs

@@ -18,7 +18,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Returns the natural logarithm. Base: (math e)"
+        "Returns the natural logarithm. Base: (math e)."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/math/max.rs

@@ -22,7 +22,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Returns the maximum of a list of numbers, or of columns in a table"
+        "Returns the maximum of a list of numbers, or of columns in a table."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/math/median.rs

@@ -24,7 +24,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Computes the median of a list of numbers"
+        "Computes the median of a list of numbers."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/math/min.rs

@@ -22,7 +22,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Finds the minimum within a list of numbers or tables"
+        "Finds the minimum within a list of numbers or tables."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/math/mode.rs

@@ -48,7 +48,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Returns the most frequent element(s) from a list of numbers or tables"
+        "Returns the most frequent element(s) from a list of numbers or tables."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/math/product.rs

@@ -19,7 +19,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Returns the product of a list of numbers or the products of each column of a table"
+        "Returns the product of a list of numbers or the products of each column of a table."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/math/round.rs

@@ -27,7 +27,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Returns the input number rounded to the specified precision"
+        "Returns the input number rounded to the specified precision."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/math/sqrt.rs

@@ -18,7 +18,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Returns the square root of the input number"
+        "Returns the square root of the input number."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/math/stddev.rs

@@ -24,7 +24,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Returns the standard deviation of a list of numbers, or of each column in a table"
+        "Returns the standard deviation of a list of numbers, or of each column in a table."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/math/sum.rs

@@ -19,7 +19,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Returns the sum of a list of numbers or of each column in a table"
+        "Returns the sum of a list of numbers or of each column in a table."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/math/variance.rs

@@ -23,7 +23,7 @@ impl Command for SubCommand {
     }
 
     fn usage(&self) -> &str {
-        "Returns the variance of a list of numbers or of each column in a table"
+        "Returns the variance of a list of numbers or of each column in a table."
     }
 
     fn search_terms(&self) -> Vec<&str> {