ctdb-doc: Document that "ctdb tickle" can now read from stdin

ctdb-doc: Document that "ctdb tickle" can now read from stdin

BUG: https://bugzilla.samba.org/show_bug.cgi?id=12123

Although this has been queued for a while, with changes to add stdin
support to the original ctdb tool, this came for free with the
replacement tool.

addtickle and deltickle also now have this feature.  However, they're
internal commands and we're not adding new documentation for
internal/debug commands.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>