(docs) Revise and unify explanations of module/file references in function docs
(docs) Mention file() function in file type's content attribute

Now that this function works in a more useful way, we can advertise it a bit
more widely.
Merge pull request #3016 from nfagerlund/docs_37_file_function

(docs) Revise and unify explanations of module/file references in functi...