Cheap doc fixes

Besides the XML hangovers, I noticed a few other issues in the manual. I've fixed a few of them here but the manual could stand a thorough going-over.

In particular, the +plusses+ are used throughout the manual, yet since AsciiDoctor 1.5 they don't do what we want — they only suppress format parsing, when usually we're after the monospace font which now requires `backticks`. If it would be useful, I could go through the whole manual fixing these (and other bobbles while I'm at it), but I thought I should submit these initial fixes first to check I'm not barking up the wrong tree.

There is one place where I make a semantic change: under the unmerge command I have changed the reparent equivalent to use SVO syntax. I am not 100% sure I have done this correctly.

I was not sure what the difference was between <link> and <xref> in the old DocBook, so I translated them both to <<>>.