NFSD: Flesh out a documenting comment for filecache.c

[ Upstream commit b3276c1f ]

Record what we've learned recently about the NFSD filecache in a
documenting comment so our future selves don't forget what all this
is for.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>