(maint) PUP-3638 clarify puppet-doc manpage re: rdoc on manifests

remove the manifest examples and related doc
this functionality has been removed in PUP-3638
Merge pull request #3381 from er0ck/maint/master/PUP-3638-remove_manifest_rdoc_from_doc_manpage

(maint) PUP-3638 clarify puppet-doc manpage re: rdoc on manifests