(DOCSP-30026) Update dbstats with info about 20 min intervals (#5065) (#5068)

Author: JuliaMongo

source/reference/monitoring.txt

@@ -19,7 +19,8 @@ of your MongoDB deployment. The agent uses these commands:
 
 - :dbcommand:`connPoolStats`
 
-- :dbcommand:`dbStats`
+- :dbcommand:`dbStats` The {+magent+} collects storage statistics for a
+  given database at 20 minute intervals.
 
 - :dbcommand:`find` on the following :manual:`config database </reference/config-database>`
   collections, which support sharding operations: