docs: make the guillemot and the middle dot appear correctly in HTML

For the man page, encode them properly, and for the Info stuff,
just set the right document encoding.

This fixes https://savannah.gnu.org/bugs/?51369.

doc/nano.texi

@@ -4,6 +4,8 @@
 @settitle nano
 @c %**end of header
 
+@documentencoding UTF-8
+
 @smallbook
 @set EDITION 0.4
 @set VERSION 2.8.5

doc/nanorc.5

@@ -271,7 +271,7 @@ Disallow file modification.
 .B set whitespace "\fIstring\fP"
 Set the two characters used to indicate the presence of tabs and
 spaces.  They must be single-column characters.  The default pair
-for a UTF-8 locale is "\fB»·\fR", and for other locales "\fB>.\fR".
+for a UTF-8 locale is "\fB\[Fc]\[md]\fR", and for other locales "\fB>.\fR".
 .TP
 .B set wordbounds
 Detect word boundaries differently by treating punctuation