(DOCS-16471) getField supports strings that are not string literal (#5197)

Author: sarahsimpers

source/reference/operator/aggregation/getField.txt

@@ -56,12 +56,11 @@ Syntax
      - String
      - Field in the ``input`` object for which you want to return a 
        value. ``field`` can be any valid :ref:`expression 
-       <aggregation-expressions>` that resolves to a string 
-       constant.
+       <aggregation-expressions>` that resolves to a string.
 
        If ``field`` begins with a dollar sign (``$``), place the field
-       name inside of a :expression:`$literal` expression to return its
-       value.
+       name inside of a :expression:`$literal` or
+       :expression:`$toString` expression to return its value.
 
    * - ``input``
      - Object
@@ -89,9 +88,6 @@ described above.
 Behavior
 --------
 
-- If ``field`` resolves to anything other than a string constant,
-  :expression:`$getField` returns an error.
-
 - If the ``field`` that you specify is not present in the ``input`` 
   object, or in :variable:`$$CURRENT <CURRENT>` if you don't specify an
   ``input`` object, :expression:`$getField` returns ``missing``.