doc: add-or-rm-mons.rst: better explanation on number of monitors

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
doc: troubleshooting-mons.rst: better document how to troubleshoot mons

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge pull request #1125 from ceph/wip-mon-docs

wip-mon-docs: Better explain required number of monitors & how to troubleshoot a monitor

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Loic Dachary <loic@dachary.org>