Document default locale in MockHttpServletRequest

This commit updates the class-level Javadoc for MockHttpServletRequest
with information regarding the default locale for the mocked server.

Issue: SPR-11701
Document that mock request doesn't support Accept-Language header

This commit updates the Javadoc for getLocale() and getLocales() in
MockHttpServletRequest to point out that the mock implementation does
not comply with the the Servlet specification with regard to the
Accept-Language header.

Issue: SPR-12043