[RFA] gdbarch.sh: document REGISTER_SIZE and REGISTER_BYTES

[Michael Snyder]

Andrew Cagney wrote:

> Given that pseudo-register values are created using a fuction and raw
> register data, do they have any storage?

Sometimes yes, sometimes no.  A pseudo-register can be just
an alias for another register, in which case it can share 
storage.  But it can also be a computed value, in which case
it may have its own storage.  I had one case in which a 
pseudo-register was the concatenation of two regular
registers -- but they weren't consecutive, so I had to
give the pseudo its own storage and copy them.