An earlier optimization of ansible-doc -l caused failures. (#47012)

* An earlier optimization of ansible-doc -l caused failures.

The optimization quickly searches the plugin code for short_description
fields and then uses that in the -l output.  The searching was a bit too
naive and ended up pulling out malformed yaml.  This caused those
plugins to be omitted from the list of plugins of that type with
a warning that their documentation strings were wrong.

This change makes the documentation parser aware that the documentation
string could have a relative indent for all of its fields which makes it
robust in the face of this particular problem.

* Don't search for space after short_description:

Any whitespace would be valid.  In particular newline

changelogs/fragments/plugin-docs-list-fix.yaml

@@ -0,0 +1,3 @@
+---
+bugfixes:
+- 'Fixed an issue with ansible-doc -l failing when parsing some plugin documentation.'