Documentation: adjust to AsciiDoc 8

Documentation: adjust to AsciiDoc 8

It turns out that the attribute definition we have had for a
long time to hide "^" character from AsciiDoc 7 was not honored
by AsciiDoc 8 even under "-a asciidoc7compatible" mode.  There is
a similar breakage with the "compatible" mode with + characters.

The double colon at the end of definition list term needs
to be attached to the term, without a whitespace.  After this
minimum fixups, AsciiDoc 8 (I used 8.2.1 on Debian) with
compatibility mode seems to produce reasonably good results.

Signed-off-by: Junio C Hamano <gitster@pobox.com>