doc: convert [\--] to [--]

doc: convert [\--] to [--]

Commit 1c262bb7b (doc: convert \--option to --option, 2015-05-13)
explains that we used to need to write \--option to play well with older
versions of AsciiDoc, but that we do not support such versions anymore
anyway, and that Asciidoctor literally renders \--.

With [\--], which is used to denote the optional separator between
revisions and paths, Asciidoctor renders the backslash literally.
Change all [\--] to [--]. This changes nothing for AsciiDoc version
8.6.9, but is an improvement for Asciidoctor version 1.5.4.

We use double-dashes in several list entries (\--::). In my testing, it
appears that we do need to use the backslash there, so leave those.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>