Merge pull request #5024 from cakebaker/docs_add_ls_long_to_extensions

docs: add "ls --long" to extensions

docs/src/extensions.md

@@ -69,3 +69,8 @@ number of spaces representing a tab when determining the line length.
 ## `seq`
 
 `seq` provides `-t`/`--terminator` to set the terminator character.
+
+## `ls`
+
+GNU `ls` provides two ways to use a long listing format: `-l` and `--format=long`. We support a
+third way: `--long`.