Clarify single field index management in Firestore docs

[eddavisson]

firestore: clarified which resource to use for which kind of index in Standard and Enterprise editions

[github-actions]

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit a66df1f:

Diff report

Your PR generated the following diffs in downstream repositories:

Repository	Diff Link	Changes
google provider	View Diff	3 files changed, 18 insertions(+), 14 deletions(-)
google-beta provider	View Diff	3 files changed, 18 insertions(+), 14 deletions(-)

Test report

Analytics

Total Tests	Passed	Skipped	Affected
40	40	0	0

Affected Service Packages

• firestore

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@eddavisson, @SirGitsalot VCR tests complete for a66df1f!

[bw-yuhogg]

Approving on behalf of Firestore