Rename $usableSkins to $allowedSkins

Rename $usableSkins to $allowedSkins

This change is entirely in local scope.
The function is still called getUsableSkins()
so this change won't break anything.
But the variable name and the updated documentation comment
make it clearer what the purpose of this function is,
for anyone reading the code or reading the autogenerated docs.

This change also includes minor changes to documentation
in skin-related files (typos and small edits for clarity).

Change-Id: I4220408f4d3b64cf87be4ad1af4b72d3a4524922