Docs: Explicitly note solidityKeccak performs a packed encoding.

[gnattishness]

What was wrong?

The docs diddn't explicitly state what type of encoding web3.solidityKeccak performs.
As of Solidity v0.5.x, keccak256 doesn't do any automatic encoding.
People less familiar with Solidity v0.4.x could find the description ambiguous.

How was it fixed?

Added to relevant docs section.

I also thought it'd be nice to add examples of the equivalent solidity (which should be valid for v0.5.x - v0.8.x at least)

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Thanks! Let me know if there's any quibbles or things I can adjust.

[wolovim]

Thanks for the additional context! I'll add a newsfragment and any other wordsmithing, then merge it in.