add syntax for checking versions
docs for ninja_required_version

(And some extra docs for top-level variables in general.)
depfile helper: check errors on writing .d files

Closes #492 and #493.
say where the default value for -j comes from

This is a FAQ.
move from asciidoc html to asciidoc -> docbook -> html

As best as I can tell this is the easiest way to customize
the asciidoc HTML output.
rearrange env variable section layout