src/common: add guidance for deep-scrubbing ratio warning

Add an explanation of how to set the value of
"mon_warn_pg_not_deep_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.

Signed-off-by: Zac Dover <[email protected]>

Author: zdover23

src/common/options/global.yaml.in

@@ -1819,7 +1819,7 @@ options:
 - name: mon_warn_pg_not_deep_scrubbed_ratio
   type: float
   level: advanced
-  desc: Percentage of the deep scrub interval past the deep scrub interval to warn
+  desc: Percentage of the deep scrub interval past the deep scrub interval to warn - Set this configurable with the command "ceph config set mgr mon_warn_pg_not_deep_scrubbed_ratio <ratio_value>"
   default: 0.75
   see_also:
   - osd_deep_scrub_interval