Documentation: render special characters correctly

Documentation: render special characters correctly

Three hyphens are rendered verbatim, so "--" has to be used to produce a
dash.  There is no double arrow ("<->" is rendered as "<→"), so a left
and right arrow "<-->" have to be combined for that.

So fix asciidoc output for special characters.  This is similar to fixes
in commit de82095a95 (doc hash-function-transition: fix asciidoc output,
2021-02-05).

Signed-off-by: Andrei Rybak <redacted>
Acked-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>