fix google_project_service docs to mention it supports user project overrides

[drebes]

see code

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
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[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 6 insertions(+))
Terraform Beta: Diff ( 2 files changed, 7 insertions(+), 2 deletions(-))

[melinath]

/gcbrun re-running due to CI issue

[melinath]

/gcbrun - CI issue is now resolved

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 6 insertions(+))
Terraform Beta: Diff ( 2 files changed, 7 insertions(+), 1 deletion(-))