DOCS-10517: tweak - bold the word not

Author: Kay Kim

source/core/index-unique.txt

@@ -237,7 +237,7 @@ For a ranged sharded collection, only the following indexes can be
 
 - the default ``_id`` index; **however**, the ``_id`` index only
   enforces the uniqueness constraint per shard **if** the ``_id`` field
-  is not the shard key or the prefix of the shard key.
+  is **not** the shard key or the prefix of the shard key.
 
   .. admonition:: Uniqueness and the ``_id`` Index
      :class: important

source/core/sharding-shard-key.txt

@@ -104,7 +104,7 @@ For a ranged sharded collection, only the following indexes can be
 
 - the default ``_id`` index; **however**, the ``_id`` index only
   enforces the uniqueness constraint per shard **if** the ``_id`` field
-  is not the shard key or the prefix of the shard key.
+  is **not** the shard key or the prefix of the shard key.
 
   .. admonition:: Uniqueness and the ``_id`` Index
      :class: important