Add notes about user_project_override in google_firebase_project

[rainshen49]

Fixes hashicorp/terraform-provider-google#18668

Release Note Template for Downstream PRs (will be copied)

firebase: Documentation only

[github-actions]

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[modular-magician]

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

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 1 file changed, 4 insertions(+))
google-beta provider: Diff ( 1 file changed, 4 insertions(+))

[modular-magician]

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

Diff report

Your PR generated some diffs in downstreams - here they are.

google provider: Diff ( 1 file changed, 5 insertions(+), 1 deletion(-))
google-beta provider: Diff ( 1 file changed, 5 insertions(+), 1 deletion(-))

[rainshen49]

@trodge This is a trivial change. Feel free to approve without service team review.