(#2462) - Fix API documentation for cache constructor option

Updated the documentation for new PouchDB's cache option.
The flag is a sub property of the ajax option, not a top
level option.
(#2207) - provide additional information about the db.compact() method

Give some high-level information about the compaction operation itself,
highlight its availability only with the http adapter. Provide a link
to CouchDB's docs for more low-level detail.