GKE master size is not usually fix, this documentation assumes it is

[kmassada]

GKE master size is not usually fix, this documentation assumes it is, changing wording to make it more accurate. Addresses Issue #8640

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[lucperkins]

This PR accomplishes essentially the same thing as #9226 so I'm going to close it in favor of that.

Apologies for the mix-up, @kmassada! Thanks for the help!