llSetGroundTexture - no return value according to implementation

[Martin-Pitt]

The function llSetGroundTexture does not actually return anything meaningful in the implementation.

According to @HaroldCindy and Monty Linden (per SL discord #scripting) checking the code there were no return values.

If that's the case then we could tweak the definition to match with implementation.

Alternatively, this PR could be rejected and the implementation updated to match with estate management functions e.g. llManageEstateAccess:

Returns a boolean (an integer) TRUE if the call was successful; FALSE if throttled, invalid action, invalid [...] or object owner is not allowed to manage the estate.

[HaroldCindy]

This can't be changed to void because that would break API compatibility, a note should be added to explain that the retval has no meaning for now. We can update the note if / when that changes.