google: adds header to security considerations section

Change-Id: I29b93715876f233ae52687c8223fd8733a2a3b80
GitHub-Last-Rev: f15c4cf1a5
GitHub-Pull-Request: golang/oauth2#677
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/535895
Reviewed-by: Cody Oss <codyoss@google.com>
Run-TryBot: Cody Oss <codyoss@google.com>
Reviewed-by: Alex Eitzman <eitzman@google.com>
Auto-Submit: Cody Oss <codyoss@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

google/doc.go

@@ -101,6 +101,8 @@
 // executable-sourced credentials), please check out:
 // https://cloud.google.com/iam/docs/workforce-obtaining-short-lived-credentials#generate_a_configuration_file_for_non-interactive_sign-in
 //
+// # Security considerations
+//
 // Note that this library does not perform any validation on the token_url, token_info_url,
 // or service_account_impersonation_url fields of the credential configuration.
 // It is not recommended to use a credential configuration that you did not generate with