Update Firestore documentation.

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/Firestore/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:e8ac9200640e76d54643f370db71a1556bf254f565ce46b45a467bbcbacbdb37
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/firestore/artman_firestore_v1.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-firestore-v1.
synthtool > Running generator for google/firestore/artman_firestore.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/php/google-cloud-firestore-v1beta1.
synthtool > Replaced 'Copyright \\d{4}' in src/V1beta1/Gapic/FirestoreGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1beta1/FirestoreClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V1beta1/FirestoreClientTest.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/Gapic/FirestoreGapicClient.php.
synthtool > Replaced 'Copyright \\d{4}' in src/V1/FirestoreClient.php.
synthtool > Replaced 'Copyright \\d{4}' in tests/Unit/V1/FirestoreClientTest.php.
synthtool > Cleaned up 1 temporary directories.
synthtool > Wrote metadata to synth.metadata.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@ab522c1). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1743   +/-   ##
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  Coverage          ?   91.51%           
  Complexity        ?     4360           
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  Files             ?      304           
  Lines             ?    12865           
  Branches          ?        0           
=========================================
  Hits              ?    11773           
  Misses            ?     1092           
  Partials          ?        0

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