doc: Improve wording for packed IDs
"is incompatible with the implementation in libwayland" is a common source of confusion as evidenced by repeated discussions in IRC channel.
Improve the wording by making clear that
- packing IDs is a protocol requirement
- there are implementations (including libwayland) that enforce it
Signed-off-by: Mikhail Gusarov dottedmag@dottedmag.net