Adds clarifying language around pluralize and singularize

Adds a note to clarify that pluralize and singularize while in general will work with most English nouns, it is possible for them to have issues around singularizing a singular noun and same with pluralizing a plural noun.

Related CakePHP Core Issue that brought forth this change: https://github.com/cakephp/cakephp/issues/6236#issuecomment-88313571