Allow LintCode to override the default documentation URL
Allow LintCode to override the default documentation URL

We need this in order to override the default as we start to migrate the
linter docs to the new format. We also need this for internal lints.

Change-Id: I9da7be2e6c657b390ae355cdfcd1e007c70769fd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/264061
Reviewed-by: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@google.com>
Reviewed-by: Phil Quitslund <3faecbdd7b7be83f493f037bf1d597e6bb9d0848@google.com>
Commit-Queue: Brian Wilkerson <1f7641b6b14c52b9163524ab8d9aabff80176f21@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk