Switch back to ASCII for symbols.

Garrett Wollman reported privately that XEmacs 21.4.22, the current
stable version, doesn't work with the UTF-8 recently introduced
into our commentary.  For example, the UTF-8 character '−'
(MINUS SIGN), which is stored as the three bytes "\342\210\222",
displays as 'â\210\222'.  For proper names this is annoying but
tolerable, as there's little loss in utility from (say) 'Racoviță'
to its display form 'RacoviÈ\233Ä\203'.  But for symbols this is a
real pain that can make it hard to understand the documentation, e.g.,
'Release 2014e – 2014-06-12 21:53:52 −0700' displays as
'Release 2014e â\200\223 2014-06-12 21:53:52 â\210\2220700'.
To work around this problem, make the following substitutions in
commentary to mostly revert these symbols to their pre-UTF-8 versions:
'§' -> 'section', '°' -> 'degrees', '±' -> '+-', '–' -> '-' (en
dash), '—' -> '--' (em dash), '′' -> "'", '″' -> '"', '→' -> '->',
'−' -> '-' (minus sign), '≤' -> '<='.  Leave proper names and
foreign words in UTF-8.