Adjust examples in date commands (#12055)

Hello! This is my first PR to nushell, as I was looking at things for
#5066. The usage text for the date commands seemed fine to me, so this
is just a bit of a tidy up of the examples, mostly the description text.

# Description

- Remove two incorrect examples for `date to-record` and `date to-table`
where nothing was piped in (which causes an error in actual use).

- Fix misleading descriptions in `date to-timezone` which erroneously
referred to Hawaii's time zone.

- Standardise on "time zone" in written descriptions.

- Generally tidy up example descriptions and improve consistency.

# User-Facing Changes

Only in related help text showing examples.

crates/nu-command/src/date/list_timezone.rs

@@ -51,7 +51,7 @@ impl Command for SubCommand {
     fn examples(&self) -> Vec<Example> {
         vec![Example {
             example: "date list-timezone | where timezone =~ Shanghai",
-            description: "Show timezone(s) that contains 'Shanghai'",
+            description: "Show time zone(s) that contains 'Shanghai'",
             result: Some(Value::test_list(vec![Value::test_record(record! {
                 "timezone" => Value::test_string("Asia/Shanghai"),
             })])),

crates/nu-command/src/date/now.rs

@@ -46,17 +46,17 @@ impl Command for SubCommand {
                 result: None,
             },
             Example {
-                description: "Get the time duration from 2019-04-30 to now",
+                description: "Get the time duration since 2019-04-30.",
                 example: r#"(date now) - 2019-05-01"#,
                 result: None,
             },
             Example {
-                description: "Get the time duration since a more accurate time",
+                description: "Get the time duration since a more specific time.",
                 example: r#"(date now) - 2019-05-01T04:12:05.20+08:00"#,
                 result: None,
             },
             Example {
-                description: "Get current time in full RFC3339 format with timezone",
+                description: "Get current time in full RFC 3339 format with time zone.",
                 example: r#"date now | debug"#,
                 result: None,
             },

crates/nu-command/src/date/to_record.rs

@@ -50,11 +50,6 @@ impl Command for SubCommand {
 
     fn examples(&self) -> Vec<Example> {
         vec![
-            Example {
-                description: "Convert the current date into a record.",
-                example: "date to-record",
-                result: None,
-            },
             Example {
                 description: "Convert the current date into a record.",
                 example: "date now | date to-record",

crates/nu-command/src/date/to_table.rs

@@ -52,11 +52,6 @@ impl Command for SubCommand {
         vec![
             Example {
                 description: "Convert the current date into a table.",
-                example: "date to-table",
-                result: None,
-            },
-            Example {
-                description: "Convert the date into a table.",
                 example: "date now | date to-table",
                 result: None,
             },

crates/nu-command/src/date/to_timezone.rs

@@ -77,27 +77,27 @@ impl Command for SubCommand {
 
         vec![
             Example {
-                description: "Get the current date in UTC+05:00",
+                description: "Get the current date in UTC+05:00.",
                 example: "date now | date to-timezone '+0500'",
                 result: None,
             },
             Example {
-                description: "Get the current local date",
+                description: "Get the current date in the local time zone.",
                 example: "date now | date to-timezone local",
                 result: None,
             },
             Example {
-                description: "Get the current date in Hawaii",
+                description: "Get the current date in Hawaii.",
                 example: "date now | date to-timezone US/Hawaii",
                 result: None,
             },
             Example {
-                description: "Get the current date in Hawaii",
+                description: "Get a date in a different time zone, from a string.",
                 example: r#""2020-10-10 10:00:00 +02:00" | date to-timezone "+0500""#,
                 result: example_result_1(),
             },
             Example {
-                description: "Get the current date in Hawaii, from a datetime object",
+                description: "Get a date in a different time zone, from a datetime.",
                 example: r#""2020-10-10 10:00:00 +02:00" | into datetime | date to-timezone "+0500""#,
                 result: example_result_1(),
             },