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>