DOCS-13797 limit simultaneous index builds (#1649)

* DOCS-13797 limit simultaneous index builds

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Author: Dave Cuthbert

source/reference/parameters.txt

@@ -1241,6 +1241,39 @@ General Parameters
 
       mongod --setParameter failIndexKeyTooLong=false
 
+
+.. parameter:: maxNumActiveUserIndexBuilds
+
+   |mongod-only|
+
+   *Type*: integer
+
+   *Default*: 3
+
+   MongoDB limits the number of concurrent user index builds to control
+   WiredTiger cache pressure. The primary node can run up to
+   ``maxNumActiveUserIndexBuilds`` concurrent user index builds. This is
+   a global limit that applies across collections.
+   
+   System indexes are not limited to ``maxNumActiveUserIndexBuilds``,
+   however a running system index build will count against the limit for
+   user index builds.
+
+   After reaching ``maxNumActiveUserIndexBuilds``, MongodDB blocks
+   additional user index builds until the number of concurrent index
+   builds drops below the ``maxNumActiveUserIndexBuilds`` limit. If an
+   index build is blocked, the server logs a message, "Too many index
+   builds running simultaneously, waiting until the number of active
+   index builds is below the threshold".
+
+   The following command sets a limit of 4 concurrent index builds:
+
+   .. code-block:: javascript
+
+      db.adminCommand( { setParameter: 1, maxNumActiveUserIndexBuilds: 4 } )
+
+   See also: :parameter:`maxIndexBuildMemoryUsageMegabytes`
+
 .. parameter:: notablescan
 
    |mongod-only|