DOCS-958: Clarification on $regex use of indexes

Author: stennie

source/reference/operator/regex.txt

@@ -73,10 +73,11 @@ $regex
    ``collection`` that match the case insensitive regular expression
    ``acme.*corp`` that *don't* match ``acmeblahcorp``.
 
-   :operator:`$regex` uses :term:`indexes <index>` only when the
-   regular expression has an anchor for the beginning (i.e. ``^``)
-   of a string. Additionally, while ``/^a/``, ``/^a.*/``, and
-   ``/^a.*$/`` are equivalent, they have different performance
-   characteristics. All of these expressions use an index if an
-   appropriate index exists; however, ``/^a.*/``, and ``/^a.*$/``
-   are slower. ``/^a/`` can stop scanning after matching the prefix.
+   :operator:`$regex` can only use an :term:`index <index>` efficiently
+   when the regular expression has an anchor for the beginning (i.e. ``^``)
+   of a string and is a case-sensitive match.  Additionally, while
+   ``/^a/``, ``/^a.*/``, and ``/^a.*$/`` match equivalent strings, they
+   have different performance characteristics. All of these expressions
+   use an index if an appropriate index exists; however, ``/^a.*/``,
+   and ``/^a.*$/`` are slower. ``/^a/`` can stop scanning after matching
+   the prefix.