DOCS-13943 Add plan cache limit and stats

Author: npentrel

source/core/query-plans.txt

@@ -72,6 +72,29 @@ The plan cache does not persist if a :binary:`~bin.mongod` restarts or shuts dow
    entire plan cache. Users can also clear particular plan cache entries using
    :method:`PlanCache.clearPlansByQuery()`.
 
+Plan Cache Debug Info Size Limit
+--------------------------------
+
+Starting in 4.0.23 (and 3.6.23), the
+:doc:`plan cache </core/query-plans>` will save full ``plan cache``
+entries only if the cumulative size of the ``plan caches`` for all
+collections is lower than 0.5 GB. When the cumulative size of the
+``plan caches`` for all collections exceeds this threshold, additional
+``plan cache`` entries are stored with stripped debug information.
+
+When ``plan cache`` entries are stored with stripped debug information,
+the :dbcommand:`planCacheListQueryShapes` command output displays an
+empty document in the ``shapes`` array for each each plan cache entry for
+which debug information was stripped. Similarly, detailed execution
+statistics for the candidate plans are omitted from the
+:dbcommand:`planCacheListPlans` command output when ``plan cache``
+entries are stored with stripped debug information. When debug info is
+present, you can find these detailed statistics in the ``plans.reason``
+field.
+
+The estimated size in bytes of a ``plan cache`` entry is available in
+the output of :dbcommand:`planCacheListPlans`.
+
 .. _index-filters:
 
 Index Filters