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
Merge "Rename $usableSkins to $allowedSkins"