[COLLECTIONS-732] Document that MultiValuedMap.mapIterator() does not support setValue()

[sharanggupta]

Fixes COLLECTIONS-732.

What

MultiValuedMap.mapIterator() returns an iterator whose setValue(Object) always throws UnsupportedOperationException (the MultiValuedMapEntry instances it iterates are read-only), but neither the interface Javadoc nor the implementation said so, which is what the ticket asked for; @kinow invited a Javadoc PR there in 2020.

How

Javadoc only, no code change:

• MultiValuedMap#mapIterator(): says that the iterator visits one key-value mapping at a time, that the implementations in this library do not support MapIterator#setValue(Object) and throw UnsupportedOperationException from it, and how to replace a value instead (remove()/removeMapping + put).
• AbstractMultiValuedMap#mapIterator(): {@inheritDoc} plus the same note for this implementation.
• AbstractMultiValuedMap.MultiValuedMapIterator#setValue(Object): documents the IllegalStateException before next() and the UnsupportedOperationException otherwise.

The behaviour itself is already pinned by AbstractMultiValuedMapTest.testMapIteratorUnsupportedSet, so no test was added. Verified locally with the default mvn goal on Java 17 (full test suite: 22,257 tests, plus RAT, Javadoc, japicmp, Checkstyle and PMD).

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[kinow]

I've allowed GH Actions to run. I haven't looked at the API lately, so I can do a review but it will take a few days until I have time to sync the code and read it. @garydgregory feel free to review it before if you'd like 👍

Thanks for the PR @sharanggupta !