Mention that escaping "^" as "\(ha" in source code samples improves

Mention that escaping "^" as "\(ha" in source code samples improves
portability, in particular when rendering to formats such as PDF
with real typesetters such as groff.

While here, make it even more explicit that the rendering of raw
ASCII accents ( ` ' ~ ^ ) found in the input is kind of a mess and
varies among implementations, software versions, macro sets, and
output devices.

Issue noticed when reading the groff_char(7) manual, and confirmed
by testing with current groff.

OK bentley@, "certainly no objection" jmc@

N.B.: Nobody will get scolded for ignoring this subtlety.