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 <rybak.a.v@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fetch.txt

@@ -133,7 +133,7 @@ remember to run that, set `fetch.prune` globally, or
 linkgit:git-config[1].
 
 Here's where things get tricky and more specific. The pruning feature
-doesn't actually care about branches, instead it'll prune local <->
+doesn't actually care about branches, instead it'll prune local <-->
 remote-references as a function of the refspec of the remote (see
 `<refspec>` and <<CRTB,CONFIGURED REMOTE-TRACKING BRANCHES>> above).
 

Documentation/gittutorial.txt

@@ -322,7 +322,7 @@ initiating this "pull".  If Bob's work conflicts with what Alice did since
 their histories forked, Alice will use her working tree and the index to
 resolve conflicts, and existing local changes will interfere with the
 conflict resolution process (Git will still perform the fetch but will
-refuse to merge --- Alice will have to get rid of her local changes in
+refuse to merge -- Alice will have to get rid of her local changes in
 some way and pull again when this happens).
 
 Alice can peek at what Bob did without merging first, using the "fetch"