DOCSP-26654: Bolding edits (#2192)

Author: sarah-olson-mongodb

source/core/sharding-balancer-administration.txt

@@ -59,8 +59,8 @@ The :term:`balancer` attempts to minimize the impact by:
 
   See also :ref:`range-migration-queuing`.
 
-- Starting a balancing round **only** when the difference in the amount 
-  of data between the shard with the most data for a sharded collection 
+- Starting a balancing round only when the **difference in the amount 
+  of data** between the shard with the most data for a sharded collection 
   and the shard with the least data for that collection reaches the 
   :ref:`migration threshold <sharding-migration-thresholds>`.