Document exp_descriptor.op_name should never return NULL.

Document exp_descriptor.op_name should never return NULL.

This documents a constaint that struct exp_descriptor's "op_name"
method implementation should obey.  This might not have been part
of the initial design, but is currently true of all instantiations,
and already assumed by the current users.

gdb/ChangeLog:

        * parser-defs.h (struct exp_descriptor): Document constraint
        on return value for "op_name" callbacks.