doc: Review man page synopses

doc: Review man page synopses

Fix use of font typefaces:

- *bold* for terminals
- 'italic' for non-terminals
- plain for meta-characters

Apart from that:

* Variable definitions require an equals sign
* 'auto-merge' option in set spec does not take a parameter
* List header fields in payload expressions instead of unexplained
  placeholder
* Introduce non-terminals in some places to avoid repetitions or clarify
  syntax
* Fix syntax for ip6 header expresssion example
* Reorganize ct expression synopsis into four parts:
  1) direction not allowed
  2) direction optional
  3) direction mandatory
  4) direction and family mandatory
* Add missing 'version' keyword to osf expression
* Clarify verdict statements example topic
* Add synopses for payload and exthdr statements
* Fix typo: differv -> diffserv
* Reorganize reject statement synopsis to point out which code type
  is required for which type arg
* Counter statement requires either one of 'packets' or 'bytes' args or
  both, none is an invalid variant
* Limit statement accepts a unit in burst, too
* Improve language in limit statement description a bit

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>