Documentation: quote {non-attributes} for asciidoc

Documentation: quote {non-attributes} for asciidoc

Asciidoc treats {foo} as an attribute to be substituted; if
'foo' doesn't exist as an attribute, then the entire line
gets dropped. When the literal {foo} is desired, \{foo} is
required.

The exceptions to this rule are:
  - inside literal blocks
  - if the 'foo' contains non-alphanumeric characters (e.g.,
    {foo|bar} is assumed not to be an attribute)

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>