Documentation: turn middle-of-line tabs into spaces

Documentation: turn middle-of-line tabs into spaces

These tabs happen to appear in columns where they don't stand out too
much, so the diff here is non-obvious. Some of these are rendered
differently by AsciiDoc and Asciidoctor (although the difference might
be invisible!), which is how I found a few of them. The remainder were
found using `git grep "[a-zA-Z.,)]$TAB[a-zA-Z]"`.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>