btrfs: update some folio related comments

Remove references to the page lock and page->mapping.  Also btrfs folios
can never be swizzled into swap (mentioned in extent_write_cache_pages()).

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>