doc: use symbolic names for chain priorities

doc: use symbolic names for chain priorities

This replaces the numbers with the matching symbolic names with one
exception: The NAT example used "priority 0" for the prerouting
priority. This is replaced by "dstnat" which has priority -100 which is
the new recommended priority.

Also use spaces instead of tabs for consistency in lines which require
updates.

Signed-off-by: Simon Ruderich <simon@ruderich.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>