chore(firestore): Clarify Transaction.Get error code

[ordovicia]

Clarifies that Transaction.Get() returns an error with NotFound status code.

DocumentRef.Get() comment already states that it returns NotFound error.

google-cloud-go/firestore/docref.go

Lines 66 to 71 in 06721e0

	// Get retrieves the document. If the document does not exist, Get return a NotFound error, which
	// can be checked with
	// status.Code(err) == codes.NotFound
	// In that case, Get returns a non-nil DocumentSnapshot whose Exists method return false and whose
	// ReadTime is the time of the failed read operation.
	func (d *DocumentRef) Get(ctx context.Context) (_ *DocumentSnapshot, err error) {

Fixes #6174

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[ordovicia]

@enocom @telpirion Please take a look. Thank you.

[telpirion]

@ordovicia Please create a GitHub issue to track your request, per CONTRIBUTING guidelines. Thanks!

[ordovicia]

Ah, sorry. I created an issue #6174.

[ordovicia]

@telpirion @enocom Hi, please take a look again. Thank you.

[ordovicia]

@telpirion @enocom Ping?

[ordovicia]

@telpirion @enocom Ping?

[ordovicia]

@telpirion @enocom Ping?

[enocom]

@ordovicia Sorry for the slow response. This got lost in all my other GitHub emails. Thanks for the docs update!