tools/pfilter-builder: update/reindent comments

Actually, the "new" information is very little.  But now the
explanation of how it works fits in 80 columns (I personally need
this) and is laid out a little better.  Also, it describes the new
situation, activated by the next commits.

No code change at this point.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>