Add RETURN docs for the pip module (#39907)

The pip module returns some things. Let's document them.

lib/ansible/modules/packaging/language/pip.py

@@ -190,6 +190,34 @@ EXAMPLES = '''
   become: True
 '''
 
+RETURN = '''
+cmd:
+  description: pip command used by the module
+  returned: success
+  type: string
+  sample: pip2 install ansible six
+name:
+  description: list of python modules targetted by pip
+  returned: success
+  type: list
+  sample: ['ansible', 'six']
+requirements:
+  description: Path to the requirements file
+  returned: success, if a requirements file was provided
+  type: string
+  sample: "/srv/git/project/requirements.txt"
+version:
+  description: Version of the package specified in 'name'
+  returned: success, if a name and version were provided
+  type: string
+  sample: "2.5.1"
+virtualenv:
+  description: Path to the virtualenv
+  returned: success, if a virtualenv path was provided
+  type: string
+  sample: "/tmp/virtualenv"
+'''
+
 import os
 import re
 import sys