gen-art-25

- "the generation counters should" -> "the generation counters
  SHOULD"
- "An implementation could use." -> "An implementation MAY use"
- "the node confirms that the generation counter matches" -> "the
  node MUST confirm that the generation counter matches"
- "If it does not match, it is silently dropped." -> "If it does not
  match, it MUST be silently dropped."

Closes deanwillis/p2psip-base-master#158