[stable-2.9] User - correct group removal description in docs (#61936)

(cherry picked from commit 61efffcbe1)

Co-authored-by: Sam Doran <sdoran@redhat.com>

changelogs/fragments/user-docs-group-fix.yaml

@@ -0,0 +1,2 @@
+bugfixes:
+  - user - update docs to reflect proper way to remove account from all groups

lib/ansible/modules/system/user.py

@@ -57,8 +57,7 @@ options:
     groups:
         description:
             - List of groups user will be added to. When set to an empty string C(''),
-              C(null), or C(~), the user is removed from all groups except the
-              primary group. (C(~) means C(null) in YAML)
+              the user is removed from all groups except the primary group.
             - Before Ansible 2.3, the only input format allowed was a comma separated string.
             - Mutually exclusive with C(local)
         type: list