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/hash/generic_digest.rs

@@ -20,7 +20,7 @@ impl<D: HashDigest> Default for GenericDigest<D> {
     fn default() -> Self {
         Self {
             name: format!("hash {}", D::name()),
-            usage: format!("hash a value using the {} hash algorithm", D::name()),
+            usage: format!("Hash a value using the {} hash algorithm", D::name()),
             phantom: PhantomData,
         }
     }