doc: fix asciidoc format compatibility in pretty-formats.adoc

Asciidoc.py and Asciidoctor do not process the '+' verbatim the same way. A
span is detected when the format sign (here '+')is preceded by a non-word
character. It seems that '{nbsp}' is considered a non-word sign by
Asciidoc.py, but not by Asciidoctor.

Using a double format-sign opens 'unconstrained' span, independent on the
preceding character in both engines.

The '+' sign is used instead of the backtick '`' because it is not processed
as synopsis in asciidoc.py. Unfortunately, the post-processing of verbatim
synopsis in asciidoctor cannot be bypassed and formatting of the parentheses
is forced in syntax sign instead of keywords, unless a proper grammar
analyzer is used.

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.adoc

@@ -233,11 +233,11 @@ colon and zero or more comma-separated options. Option values may contain
 literal formatting codes. These must be used for commas (`%x2C`) and closing
 parentheses (`%x29`), due to their role in the option syntax.
 +
-** `prefix=<value>`: Shown before the list of ref names.  Defaults to "{nbsp}+(+".
+** `prefix=<value>`: Shown before the list of ref names.  Defaults to "{nbsp}++(++".
 ** `suffix=<value>`: Shown after the list of ref names.  Defaults to "+)+".
 ** `separator=<value>`: Shown between ref names.  Defaults to "+,+{nbsp}".
 ** `pointer=<value>`: Shown between HEAD and the branch it points to, if any.
-		      Defaults to "{nbsp}+->+{nbsp}".
+		      Defaults to "{nbsp}++->++{nbsp}".
 ** `tag=<value>`: Shown before tag names. Defaults to "`tag:`{nbsp}".
 
 +