Documentation: dev-tools: clarify KTAP specification wording

Documentation: dev-tools: clarify KTAP specification wording

Add the spec version to the title line.

Explain likely source of "Unknown lines".

"Unknown lines" in nested tests are optionally indented.

Add "Unknown lines" items to differences between TAP & KTAP list

Convert "Major differences between TAP and KTAP" from a bullet list
to a table.  The bullet list was being formatted as a single
paragraph.

Reviewed-by: Tim Bird <Tim.Bird@sony.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Link: https://lore.kernel.org/r/20220210233630.3304495-1-frowand.list@gmail.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>