qemu-img: clarify src_cache option documentation

qemu-img: clarify src_cache option documentation

The source cache option takes the same values as the cache option.  The
documentation reads a little strange because it starts with "In contrast
the src_cache option ...".  The fact that this is comparing with the
previous documented option (the 'cache' option) is implicit.  Readers
may be confused, especially if they jump to src_cache without reading
cache documentation first.

Suggested-by: Jeff Nelson <jenelson@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>