Updated migrate_traffic description in google_app_engine_service_split_traffic resource

[thedmeyer]

Updating description of the migrate_traffic variable to be more clear on it's purpose and be in line with GCP documentation.

See: https://cloud.google.com/appengine/docs/admin-api/migrating-splitting-traffic

[pjeannet]

Hello, found this PR while looking at existing issues with migrate_traffic. You could also add that it won't work with flex environment (I tried to use it and got Traffic migration does not support App Engine Flexible Environment.).
I also noticed that it's not really optionnal as if we don't set it to false then we get Invalid value at 'migrate_traffic' (TYPE_BOOL), \"\".

[nat-henderson]

Hi there - this is correct, but as you can see at the top of this file, this code is autogenerated. Would you mind making the change at the source? That's https://github.com/GoogleCloudPlatform/magic-modules/blob/master/mmv1/products/appengine/api.yaml#L1437.