Document WebGPU edge case behaviour of bitfield operations

[dneto0]

From gpuweb/gpuweb#288

Implementation behaviour edge cases for OpBitFieldSExtract, OpBitFieldUExtract and OpBitFieldInsert are:

• Clamp Offset to [0,31] (make the bit width in bounds)
• Clamp Count such that clamped_offset + count is within [0,31].
• When inserting, only the clamped_count least significant bits are used.
• When extracting, only the clamped_count least significant bits are returned