[rfa:doco] mention gdbarch_obstack_zalloc in per-module data

[Andrew Cagney]

>> +Set the uninitialized (NULL) data-pointer corresponding to @var{handle}
>> +to the non-NULL @var{pointer} value.
> 
> 
> Both `NULL's should be in @code.
> 
> Also, is it guaranteed that uninitialized pointers have a NULL value?
> In general, that's not true, but perhaps in the context of gdbarch it
> is, I just don't know.

In general no, but here, yes.  Perhaphs:

"Set the still @code{NULL} data-pointer ...."

Andrew