Add warning documenting behavior of the google_service_account_key re…

…source for certain cases where the project cannot be inferred by terraform (GoogleCloudPlatform#5301)

* Add warning documenting behavior of the google_service_account_key resource for certain cases where the project cannot be inferred by terraform

* remove warning in favor of rewording service_account_id description

* restore newline

* clarify description to include '-' wildcard substitution

* rewrite to include unique id

* remove sa name as account option from fully qualified syntax

mmv1/third_party/terraform/website/docs/r/google_service_account_key.html.markdown

@@ -82,8 +82,11 @@ resource "kubernetes_secret" "google-application-credentials" {
 The following arguments are supported:
 
 * `service_account_id` - (Required) The Service account id of the Key. This can be a string in the format
-`{ACCOUNT}` or `projects/{PROJECT_ID}/serviceAccounts/{ACCOUNT}`, where `{ACCOUNT}` is the email address or
-unique id of the service account. If the `{ACCOUNT}` syntax is used, the project will be inferred from the account.
+`{ACCOUNT}` or `projects/{PROJECT_ID}/serviceAccounts/{ACCOUNT}`. If the `{ACCOUNT}`-only syntax is used, either
+the **full** email address of the service account or its name can be specified as a value, in which case the project will
+automatically be inferred from the account. Otherwise, if the `projects/{PROJECT_ID}/serviceAccounts/{ACCOUNT}`
+syntax is used, the `{ACCOUNT}` specified can be the full email address of the service account or the service account's
+unique id. Substituting `-` as a wildcard for the `{PROJECT_ID}` will infer the project from the account.
 
 * `key_algorithm` - (Optional) The algorithm used to generate the key. KEY_ALG_RSA_2048 is the default algorithm.
 Valid values are listed at