doc: nft.8: Minor NAT STATEMENTS section review

doc: nft.8: Minor NAT STATEMENTS section review

Synopsis insinuates an IP address argument is mandatory in snat/dnat
statements although specifying ports alone is perfectly fine. Adjust it
accordingly and add a paragraph briefly describing the behaviour.

While at it, update the redirect statement description with more
relevant examples, the current one is wrong: To *only* alter the
destination port, dnat statement must be used, not redirect.

Fixes: 6908a677ba04c ("nft.8: Enhance NAT documentation")
Signed-off-by: Phil Sutter <phil@nwl.cc>