Fix command descriptions+examples (#5129)

* Fix exit usage

* Move dfr as-date* format examples to extra_usage

* Update command usage and examples

* More docs on `str trim`

Co-authored-by: sholderbach <sholderbach@users.noreply.github.com>

crates/nu-command/src/strings/detect_columns.rs

@@ -32,7 +32,7 @@ impl Command for DetectColumns {
     }
 
     fn usage(&self) -> &str {
-        "splits contents across multiple columns via the separator."
+        "Attempt to automatically split text into multiple columns"
     }
 
     fn run(