docs: Improve ->read_folio documentation

	See the file "Locking" for more details.

``read_folio``

	called by the VM to read a folio from backing store.  The folio

	will be locked when read_folio is called, and should be unlocked

	and marked uptodate once the read completes.  If ->read_folio

	discovers that it cannot perform the I/O at this time, it can

        unlock the folio and return AOP_TRUNCATED_PAGE.  In this case,

	the folio will be looked up again, relocked and if that all succeeds,

	->read_folio will be called again.

	Called by the page cache to read a folio from the backing store.

	The 'file' argument supplies authentication information to network

	filesystems, and is generally not used by block based filesystems.

	It may be NULL if the caller does not have an open file (eg if

	the kernel is performing a read for itself rather than on behalf

	of a userspace process with an open file).

	If the mapping does not support large folios, the folio will

	contain a single page.	The folio will be locked when read_folio

	is called.  If the read completes successfully, the folio should

	be marked uptodate.  The filesystem should unlock the folio

	once the read has completed, whether it was successful or not.

	The filesystem does not need to modify the refcount on the folio;

	the page cache holds a reference count and that will not be

	released until the folio is unlocked.

	Filesystems may implement ->read_folio() synchronously.

	In normal operation, folios are read through the ->readahead()

	method.  Only if this fails, or if the caller needs to wait for

	the read to complete will the page cache call ->read_folio().

	Filesystems should not attempt to perform their own readahead

	in the ->read_folio() operation.

	If the filesystem cannot perform the read at this time, it can

	unlock the folio, do whatever action it needs to ensure that the

	read will succeed in the future and return AOP_TRUNCATED_PAGE.

	In this case, the caller should look up the folio, lock it,

	and call ->read_folio again.

	Callers may invoke the ->read_folio() method directly, but using

	read_mapping_folio() will take care of locking, waiting for the

	read to complete and handle cases such as AOP_TRUNCATED_PAGE.

``writepages``

	called by the VM to write out pages associated with the