Clarify GCS URL signing limitations on GCE

[zulkifal]

The line 'If you are on Google Compute Engine, you can't generate a signed URL' wrongly implies URL generation is impossible from GCE. Adding clarification that this line only holds true when GCE is using GCE service account.

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[zulkifal]

I signed it!

[googlebot]

CLAs look good, thanks!

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So there's good news and bad news.

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[googlebot]

A Googler has manually verified that the CLAs look good.

(Googler, please make sure the reason for overriding the CLA status is clearly documented in these comments.)

[tseaver]

@zulkifal Thanks for the patch!