more exact wording of system_tmpdirs (#76587)

lib/ansible/plugins/doc_fragments/shell_common.py

@@ -34,11 +34,13 @@ options:
     version_added: "2.10"
   system_tmpdirs:
     description:
-       - "List of valid system temporary directories on the managed machine for Ansible to choose
-         when it cannot use C(remote_tmp), normally due to permission issues.  These must be world
+       - "List of valid system temporary directories on the managed machine for Ansible to validate
+         C(remote_tmp) against, when specific permissions are needed.  These must be world
          readable, writable, and executable. This list should only contain directories which the
          system administrator has pre-created with the proper ownership and permissions otherwise
          security issues can arise."
+       - When C(remote_tmp) is required to be a system temp dir and it does not match any in the list,
+         the first one from the list will be used instead.
     default: [ /var/tmp, /tmp ]
     type: list
     env: [{name: ANSIBLE_SYSTEM_TMPDIRS}]