Fix documentation.

It's true that right now the internal skin name doesn't have to be all-lowercase,
however not doing so leads to side effects such as the changing of case of i18n message keys,
if the documentation makes programmers aware that something other than all-lowercase can
be used, they should also be aware of the side effects of doing so.

Change-Id: Ib1ed192b1ba83ae864313c34b450a1151485750b
Merge "Fix documentation."