Document why MAX_JOINTS and MAX_MORPH_WEIGHTS are set

[alice-i-cecile]

This values are used to allocate buffers. Their maximum sizes depend on the GPU/platform, the current values are the ones guaranteed to work everywhere.
To allow bigger values, you would have to check against the limits of the GPU at runtime, which would mean not using consts anymore.

Originally posted by @mockersf in #15972 (review)

[victorb]

I'm not sure how it happened, since this is supposedly completed, but MAX_MORPH_WEIGHTS was never actually documented, but instead MORPH_BUFFER_SIZE was the one that got documented.