Doc comment for CaptureLazyWildcardRegexes (#10950)

I ran across this while looking at a unit test, hovered
over it, and was annoyed that I didn't get the description.

src/Build/Utilities/EngineFileUtilities.cs

@@ -32,7 +32,9 @@ static EngineFileUtilities()
             }
         }
 
-        // used by test to reset regexes
+        /// <summary>
+        /// Test only: repopulate lazy wildcard regexes from the environment.
+        /// </summary>
         internal static void CaptureLazyWildcardRegexes()
         {
             s_lazyWildCardExpansionRegexes = PopulateRegexFromEnvironment();