(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.
(docs) Clarify note about ssh_authorized_keys in resources resource type

It's not that it's deprecated, it's that it never worked.
(PUP-2914) Remove 'gem install rgen' step now that it's vendored
Merge pull request #3021 from nfagerlund/docs_clarify_resources_resources

(docs) Clarify note about ssh_authorized_keys in resources resource type
Merge pull request #3016 from nfagerlund/docs_37_file_function

(docs) Revise and unify explanations of module/file references in functi...
(packaging) Update PUPPETVERSION to 3.7.0
Merge pull request #3037 from kylog/issue/pup-2914/remove-rgen-install-step

(PUP-2914) Remove 'gem install rgen' step now that it's vendored