Document all filter and test plugins (#4597) (#4603)

* Fix/improve docs.

* Document the a_module test.

* Document the dict filter.

* Linting.

* Add more filter docs.

* More filters.

* Update BOTMETA.

* Add another plugin.

* Fix typos.

* Add explicit entries.

* Fix lookup documentation.

(cherry picked from commit f055f47161)

Co-authored-by: Felix Fontein <felix@fontein.de>

plugins/test/a_module.py

@@ -4,6 +4,39 @@
 from __future__ import (absolute_import, division, print_function)
 __metaclass__ = type
 
+DOCUMENTATION = '''
+  name: a_module
+  short_description: Test whether a given string refers to an existing module or action plugin
+  version_added: 4.0.0
+  author: Felix Fontein (@felixfontein)
+  description:
+    - Test whether a given string refers to an existing module or action plugin.
+    - This can be useful in roles, which can use this to ensure that required modules are present ahead of time.
+  options:
+    _input:
+      description: A string denoting a fully qualified collection name (FQCN) of a module or action plugin.
+      type: string
+      required: true
+'''
+
+EXAMPLES = '''
+- name: Make sure that community.aws.route53 is available
+  ansible.builtin.assert:
+    that:
+      - >
+        'community.aws.route53' is community.general.a_module
+
+- name: Make sure that community.general.does_not_exist is not a module or action plugin
+  ansible.builtin.assert:
+    that:
+      - "'community.general.does_not_exist' is not community.general.a_module"
+'''
+
+RETURN = '''
+  _value:
+    description: Whether the module or action plugin denoted by the input exists.
+    type: boolean
+'''
 
 from ansible.plugins.loader import action_loader, module_loader