[text.encoding.aliases] Add note about what isn't required

[jwakely]

Make it explicit that copyable is intended, rather than semiregular, and that the return types of begin() and end() can differ.

Also remove a FIXME comment for something that doesn't need fixing. These requirements on a type are specified in our usual form, it would be wrong to use Remarks: or Requires: here.

[jwakely]

This came up in a GCC bug report where it was suggested that the presentation of aliases_view as an apparently empty class suggests it should be default initializable. It was also questioned whether the implementation defined return types need to be the same type or not.

@cor3ntin confirmed the intent of the P1885R12 design, which is that it models copyable but not necessarily default_initializable, and models range but not necessarily common_range. Add a note to make it clear that the normative requirements are what's required, and no more.

Please say if you'd rather have an LWG issue for this. I'd be inclined to vote NAD on such an issue, but at least we'd have the issues list recording that the normative requirements are correct as written.

[cor3ntin]

I'm happy with the change