Add help warnings for path exists and path type regarding usage (#7062)

* Add help warnings to `path exists` and `path type`

* Correction

crates/nu-command/src/conversions/into/duration.rs

@@ -36,7 +36,7 @@ impl Command for SubCommand {
     }
 
     fn extra_usage(&self) -> &str {
-        "into duration does not take leap years into account and every month is calculated with 30 days"
+        "This command does not take leap years into account, and every month is assumed to have 30 days."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/path/exists.rs

@@ -38,6 +38,11 @@ impl Command for SubCommand {
         "Check whether a path exists"
     }
 
+    fn extra_usage(&self) -> &str {
+        r#"This only checks if it is possible to either `open` or `cd` to the given path.
+If you need to distinguish dirs and files, please use `path type`."#
+    }
+
     fn run(
         &self,
         engine_state: &nu_protocol::engine::EngineState,

crates/nu-command/src/path/type.rs

@@ -36,6 +36,11 @@ impl Command for SubCommand {
         "Get the type of the object a path refers to (e.g., file, dir, symlink)"
     }
 
+    fn extra_usage(&self) -> &str {
+        r#"This checks the file system to confirm the path's object type.
+If nothing is found, an empty string will be returned."#
+    }
+
     fn run(
         &self,
         engine_state: &nu_protocol::engine::EngineState,