Use Java Generics for `MultiKeyMap`-keys as method parameters

[Chrimle]

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This is purely a draft. The suggestion made by @garydgregory was to simply update the JavaDocs. In reference to: COLLECTIONS-872

[garydgregory]

@Chrimle See item 2 in the checklist.

[Chrimle]

I've assessed this, and I would say that the keys should not be of type K. The reason, is that there is ambiguity whether all keys should of the same type or not. Regardless, I believe the better solution would allow any combination of types for the keys. Hence, if generics should be used - it wouldn't be done in this way.

These changes will be reverted, and instead, all JavaDocs in MultiKeyMap will be assessed.

@garydgregory this was just a draft. In hindsight, it could've been shared on Jira instead.