Document why MAX_JOINTS and MAX_MORPH_WEIGHTS are set (#16324)

# Objective

Fixes #15974

## Solution

Add the comment from @mockersf, adapted to fix in-context.

crates/bevy_pbr/src/render/mesh_bindings.rs

@@ -8,6 +8,12 @@ use bevy_render::{
 use crate::render::skin::MAX_JOINTS;
 
 const MORPH_WEIGHT_SIZE: usize = size_of::<f32>();
+
+/// This is used to allocate buffers.
+/// The correctness of the value depends on the GPU/platform.
+/// The current value is chosen because it is guaranteed to work everywhere.
+/// To allow for bigger values, a check must be made for the limits
+/// of the GPU at runtime, which would mean not using consts anymore.
 pub const MORPH_BUFFER_SIZE: usize = MAX_MORPH_WEIGHTS * MORPH_WEIGHT_SIZE;
 
 const JOINT_SIZE: usize = size_of::<Mat4>();

crates/bevy_pbr/src/render/skin.rs

@@ -15,6 +15,12 @@ use bevy_render::{
 use bevy_transform::prelude::GlobalTransform;
 
 /// Maximum number of joints supported for skinned meshes.
+///
+/// It is used to allocate buffers.
+/// The correctness of the value depends on the GPU/platform.
+/// The current value is chosen because it is guaranteed to work everywhere.
+/// To allow for bigger values, a check must be made for the limits
+/// of the GPU at runtime, which would mean not using consts anymore.
 pub const MAX_JOINTS: usize = 256;
 
 #[derive(Component)]