Add docs links to AutoML samples

[nnegrey]

Add docs links to AutoML samples for MimeType once available.

[munkhuushmgl]

Will this link be sufficient ?

Adding docs links to sample java code like the following:

      // Get the full path of the model.
      ModelName name = ModelName.of(projectId, "us-central1", modelId);

      // For available mime types, see:
      // https://cloud.google.com/automl/docs/reference/rest/v1/projects.locations.models/predict#textsnippet
      TextSnippet textSnippet =
          TextSnippet.newBuilder()
              .setContent(content)
              .setMimeType("text/plain") // Types: text/plain, text/html
              .build();

[lesv]

As far as I can tell, yes.