doc: update porting, vfs documentation to describe mmap_prepare()

Now that we have established .mmap_prepare() as the preferred means by
which filesystems establish state upon memory mapping of a file, update the
VFS and porting documentation to reflect this.

As part of this change, additionally update the VFS documentation to
contain the current state of the file_operations struct.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Link: https://lore.kernel.org/20250723123036.35472-1-lorenzo.stoakes@oracle.com


Signed-off-by: Christian Brauner <brauner@kernel.org>