Changelog for FieldValue change

[schmidt-sebastian]

Changelog PRs are the most fun.

[codecov]

Codecov Report

Merging #1219 into mrschmidt/rewritefieldvalue will increase coverage by 0.01%.
The diff coverage is n/a.

Flag	Coverage Δ	Complexity Δ
#FirebaseFirestore	61.93% <ø> (+0.01%)	2226 <ø> (+1)	⬆️
#FirebaseFirestore_Ktx	41.17% <ø> (ø)	0 <ø> (ø)	⬇️

Impacted Files	Coverage Δ	Complexity Δ
.../com/google/firebase/firestore/UserDataWriter.java	50% <0%> (+1.92%)	16% <0%> (+1%)	⬆️

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[wilhuff]

This should be a past-tense sentence. Something like:

Changed the in-memory representation of Firestore documents to reduce memory allocation and improve performance. Users of DocumentSnapshot.getData() and DocumentSnapshot.toObject() will see the biggest improvement.

WDYT?

[schmidt-sebastian]

Changed to:

Changed the in-memory representation of Firestore documents to
reduce memory allocations and improve performance. Calls to 
`DocumentSnapshot.getData()` and `DocumentSnapshot.toObject()` will see
the biggest improvement.

I would think that all our users use these methods, so I changed "users of" to "calls to".

[google-oss-bot]

@schmidt-sebastian: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
smoke-tests	304db8f	link	/test smoke-tests

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[wilhuff]

LGTM