Add sentence about table's keys and implementation limitations

[apinski-cavium]

This should be non-controversial as it documents what is already the behavior of implementations of today. Adding this sentence just clarifies the section and if it is ok for an implementation not to implement an infinite key size or a combination of match_kinds.

Signed-off-by: Andrew Pinski [email protected]

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[mihaibudiu]

We agreed to merge this PR at the last LDWG.