Make DeclaredName lints more specific

Change the descriptions of lints reported by `DeclaredName` to be
relevant to the particular context (e.g. whether the incorrectly named
thing was a function, mixin, or variable).

Change-Id: I2191c1a3bb4f512b7d28e377eaedce4c0ec0a849
Reviewed-on: https://gerrit.causes.com/28832
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>