Docs 15433 secondary slowdown v6.0 (#1569)

* DOCS-15433 secondary slowdown

* DOCS-15433 secondary slowdown

* Review feedback

* Review feedback

Author: Dave Cuthbert

source/reference/parameters.txt

@@ -2598,9 +2598,13 @@ If you attempt to update ``disableSplitHorizonIPCheck`` at runtime,
    |mongod-only|
 
    Maximum number of threads to use to apply replicated operations in
-   parallel. Values can range from 1 to 256 inclusive. You can only set
-   :parameter:`replWriterThreadCount` at startup and cannot change this
-   setting with the :dbcommand:`setParameter` command.
+   parallel. Values can range from 1 to 256 inclusive. However, the
+   maximum number of threads used is capped at twice the number of
+   available cores.
+   
+   You can only set :parameter:`replWriterThreadCount` at startup and
+   cannot change this setting with the :dbcommand:`setParameter`
+   command.
 
    .. seealso::