DOCS-4578 Clarify global locks for index replication

Author: kallimachos

source/core/index-creation.txt

@@ -124,13 +124,14 @@ Building Indexes on Secondaries
    background. Previously all index builds on secondaries were in the
    foreground.
 
-A foreground index build on a :term:`primary` replicates as a foreground index
-build on :term:`replica set` :term:`secondaries <secondary>`. The replication
-worker acquires a global DB lock that queues reads and writes to all databases
-on the indexing server.
-
-A background index build on a primary replicates as a background index build on
-secondaries. Secondary reads are not affected.
+A foreground index build on a :term:`primary` requires a global DB lock. It
+replicates as a foreground index build on :term:`replica set`
+:term:`secondaries <secondary>`, and the replication worker takes a global DB
+lock that queues reads and writes to all databases on the indexing server.
+
+A background index build on a primary replicates as background index builds on
+secondaries. The replication worker does not take a global DB lock, and
+secondary reads are not affected.
 
 In both cases, index operations on replica set secondaries begin after the
 primary finishes building the index.