database: Similarly rename find_message_by_docid to find_document_for_doc_id

Again preferring notmuch_database_t* over Xapian::Database*.

Also, we're standardizing on "doc_id" rather than "docid" locally, (as
an analoge to "message_id"), in spite of the "Xapian::docid" name,
(which, fortunately, we can ignore and just us "unsigned int" instead).