Docs: mark global generateSessionKeys's encryptionKeys as optional

[LucienLeMagicien]

generateSessionKeys calls Message.generateSessionKeys, which calls helper.js's getPreferredAlgo, which returns config.preferredSymmetricAlgorithm when the encryptionKeys array is empty.
encryptionKeys also defaults to an empty array, and Message.generateSessionkeys's encryptionKeys is properly marked as optional, so this seems to just be an oversight from the doc.

I'm not familiar with the code or jsdoc or if there's a particular doc style, so feel free to edit!

In any case, it looks like it renders correctly:

[twiss]

👍 Thanks!