fscache: Rewrite documentation

Rewrite the fscache documentation.

Changes
=======
ver #3:
 - The volume coherency data is now an arbitrarily-sized blob, not a u64.

ver #2:
 - Put quoting around some bits of C being referred to in the docs[1].
 - Stripped the markup off the ref to the netfs lib doc[2].

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/20211130175119.63d0e7aa@canb.auug.org.au/ [1]
Link: https://lore.kernel.org/r/20211130162311.105fcfa5@canb.auug.org.au/ [2]
Link: https://lore.kernel.org/r/163819672252.215744.15454333549935901588.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/163906986754.143852.17703291789683936950.stgit@warthog.procyon.org.uk/ # v2
Link: https://lore.kernel.org/r/163967193834.1823006.15991526817786159772.stgit@warthog.procyon.org.uk/ # v3
Link: https://lore.kernel.org/r/164021585970.640689.3162537597817521032.stgit@warthog.procyon.org.uk/ # v4