src/common: add guidance for mon_warn_pg_not_scrubbed

Add an explanation of how to set the value of
"mon_warn_pg_not_scrubbed_ratio" to the confval definition of that
variable. Although this variable contains the string "mon", it is set on
the Manager. I have added a note to direct users to set this value on
the Manager.

This issue was pointed out by Petr Tlapa on Slack in late March of 2025.

This issue is part of a small project that also encompasses
ceph#62459, which is associated with commit
aeef59a

Co-authored-by: Anthony D'Atri <[email protected]>
Signed-off-by: Zac Dover <[email protected]>

Author: zdover23

src/common/options/global.yaml.in

@@ -1810,7 +1810,7 @@ options:
 - name: mon_warn_pg_not_scrubbed_ratio
   type: float
   level: advanced
-  desc: Percentage of the scrub max interval past the scrub max interval to warn
+  desc: Raise a health warning when shallow scrubs are delayed by this percentage of the shallow scrub interval. Note that this must be set at mgr or with global scope. Set this configurable with the command "ceph config set mgr mon_warn_pg_not_scrubbed_ratio <ratio_value>".
   default: 0.5
   see_also:
   - osd_scrub_max_interval