Please document better the effects of @documentencoding

[Eli Zaretskii]

AFAICS, Texinfo 5.x made a drastic change in the effects of
@documentencoding: where earlier it only affected accents (@"o etc.),
it now also affects the translation of ``..'', and also of @code,
@samp, etc. markups, when the encoding is set to UTF-8.

This significant change is not mentioned in NEWS, although the change
is backward incompatible, at least as far as the Info output is
concerned: Info readers which parse Info markup now need to be
adapted.

Moreover, the Texinfo manual barely documents the effect of
@documentencoding on @code and similar markups -- it is only mentioned
in an obscure node describing customization variables, under
'OPEN_QUOTE_SYMBOL'.  The description of @code has a cross-reference
to there; @samp, @env, address@hidden don't have even that.  This effect
should be prominently mentioned where the markups are described, and
mentioned again where @documentencoding itself is described, otherwise
there's no chance in the world users will discover that information.

TIA