Correctly examine path for relative links in file module

Fixes #7515
Fixes #7537
Check groupnames for None as well as an empty string in add_host

Fixes #7585
corrected doc example, non existing fact to the 2 it had 'merged'
Merge pull request #7595 from bcoca/doc_include_vars_fix

corrected doc example, non existing fact to the 2 it had 'merged'