doc: specify documentation encoding

* doc/bison.texi: Add '@documentencoding UTF-8'; needed since the
manual contains UTF-8 characters.  This will cause the .info files
to contain UTF-8 quotes and the like, which should be OK nowadays.
Add @documentlanguage while we're at it.

doc/bison.texi

@@ -1,6 +1,8 @@
 \input texinfo @c -*-texinfo-*-
 @comment %**start of header
 @setfilename bison.info
+@documentencoding UTF-8
+@documentlanguage en
 @include version.texi
 @settitle Bison @value{VERSION}
 @setchapternewpage odd