Clarify name documentation for mixedCase

docs/style-guide.rst

@@ -684,7 +684,7 @@ naming styles.
 * ``mixedCase`` (differs from CapitalizedWords by initial lowercase character!)
 * ``Capitalized_Words_With_Underscores``
 
-.. note:: When using abbreviations in CapWords, capitalize all the letters of the abbreviation. Thus HTTPServerError is better than HttpServerError.
+.. note:: When using initialisms in CapWords, capitalize all the letters of the initialisms. Thus HTTPServerError is better than HttpServerError. When using initialisms is mixedCase, capitalize all the letters of the initialisms, except keep the first one lower case if it is the beginning of the name. Thus xmlHTTPRequest is better than XMLHTTPRequest.
 
 
 Names to Avoid