Add documentation to the ByteArray# primetype.

Closes #21417 (closed)

I would also like to orient the developer regarding the usage of a ByteArray#. Currently I can identify two usecases:

• Passing FFI blobs to C
• Operating on binary blobs received from outside (like the network, for instance).

But I am unable to determine if they are legitimate and will not be a source of immense grief.