doc: Update documentation for the MANY_OBJECTS_PER_PG warning

doc: Update documentation for the MANY_OBJECTS_PER_PG warning

The current documentation for the MANY_OBJECTS_PER_PG warning
states that The threshold can be raised to silence the health
warning by adjusting the mon_pg_warn_max_object_skew config
option on the monitors. It seems that this is not true (at least)
since the luminous times, and this option should be adjusted on
the managers.

I encountered this problem and I spend quite sometime injecting
the mon_pg_warn_max_object_skew to the monitors, added the option
ceph.conf and restarted the monitors several times but the warning
was not going away. I had to download the code to see what's
happening and I found out this:

$ git grep -A 3 mon_pg_warn_max_object_skew src/common/options.cc
src/common/options.cc:1480:    Option("mon_pg_warn_max_object_skew", Option::TYPE_FLOAT, Option::LEVEL_ADVANCED)
src/common/options.cc-1481-    .set_default(10.0)
src/common/options.cc-1482-    .set_description("max skew few average in objects per pg")
src/common/options.cc-1483-    .add_service("mgr"),

After I restarted the ceph-mgr service, the warning went away.

Signed-off-by: Vangelis Tasoulas <vangelis@tasoulas.net>