gas: adjust handling of quotes for .irpc

The present handling of inner double quotes can lead to very strange
diagnostics. Follow one of the two possible interpretations of the doc:
@dots{} referring to possibly multiple white space separated
@var{values}, each of which may be quoted. The original implementation,
prior to 465e5617 ("PR gas/3856"), hints at the other possible
interpretation: When quoted there's only a single @var{values}, with
inner quotes taken as ordinary characters. That, however, seems overall
less useful to me.

While touching the documentation, mirror the (inverse) spelling
correction (@section line inconsistent with actual description) to .irp
as well.