doc: extend description of fib expression

Describe the input keys and the result types.
Mention which input keys are mandatory and which keys are mutually
exclusive.

Describe which hooks can be used with the various lookup modifiers
and extend the examples with more information on fib expression
capabilities.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1663
Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>

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