Clarify compressed texture entrypoints

All WebGL extensions for compressed texture formats explicitly define new allowed enums only for `compressedTex[Sub]Image2D ` entrypoints (with ETC1 not supporting partial updates).

Strictly speaking, this is not fully accurate because in WebGL 2.0 context, compressed formats can also be passed to `texStorage2d` entrypoint.

This leads to a few additional questions:
- Which formats can be used for sliced 3D and 2D arrays in WebGL 2.0? Some GLES extensions may support those.
- OpenGL ES extensions for S3TC, S3TC sRGB, RGTC, and BPTC formats extend not only `compressedTex[Sub]Image2D` entrypoints but also regular `texImage2D`. If it's not the case with WebGL, extension specs should mention this.

Whatever the answers are, we probably need more tests to cover remaining cases.

Start by reviewing the compressed texture extension specifications and the WebGL 2.0 entrypoints named in the issue: texStorage2D, compressedTex[Sub]Image2D, and texImage2D. Determine the supported formats for 3D and 2D-array textures and whether regular texImage2D is extended, then add tests covering the remaining cases and clarify the specifications.

Written by the indexing model from the issue text.