Clarify supercompression for UASTC formats

The current spec says:
> UASTC images can be supercompressed with Zstandard (`supercompressionScheme` = 2) with or without first conditioning the data with Rate-distortion Optimization.

Similar language is being added in #216 (note that RDO is not mentioned for some reason):
> UASTC HDR images can be supercompressed with Zstandard (`supercompressionScheme` = 2).

Both of those statements are not helpful because technically _any_ KTX 2.0 file (except those using BasisLZ) **can** be supercompressed with Zstandard.

The real question is: are UASTC formats limited to Zstandard, i.e., do they disallow use of Deflate? If yes, the spec should be explicit about that; if no, the quoted language should be more generic.

Start by reviewing the current UASTC wording in the specification and the related language in issue #216. Determine whether UASTC formats permit Deflate as well as Zstandard, then update the affected specification text so the permitted supercompression behavior is explicit; done means the wording no longer implies a special rule without stating it.

Written by the indexing model from the issue text.