(doc) Improve description of recurse, recurselimit, and purge attributes

The file type's `recurse` attribute didn't really explain what it did, and
didn't accurately describe the difference between `true` and `remote`. This
commit clears that all up.
Merge pull request #2591 from nfagerlund/docs_file_recurse

(doc) Improve description of recurse, recurselimit, and purge at...