Update lookup.rst (#73716)

Document that users must pass `allow_unsafe=True` as an option in the lookup to allow templating, with a note about security implications.

Co-authored-by: Alicia Cozine <879121+acozine@users.noreply.github.com>
Co-authored-by: Sandra McCann <samccann@redhat.com>

docs/docsite/rst/plugins/lookup.rst

@@ -13,6 +13,7 @@ Lookup plugins are an Ansible-specific extension to the Jinja2 templating langua
    - Lookups are executed with a working directory relative to the role or play,
      as opposed to local tasks, which are executed relative the executed script.
    - Pass ``wantlist=True`` to lookups to use in Jinja2 template "for" loops.
+   - By default, lookup return values are marked as unsafe for security reasons. If you trust the outside source your lookup accesses, pass ``allow_unsafe=True`` to allow Jinja2 templates to evaluate lookup values.
 
 .. warning::
    - Some lookups pass arguments to a shell. When using variables from a remote/untrusted source, use the `|quote` filter to ensure safe usage.