[sdk-platform-java] [ggj] generate example of ApiCallContext.withRetrySettings on client

https://github.com/googleapis/gax-java/pull/1238 introduced a new feature to the `ApiCallContext` that allows end users to supply `RetrySetttings` to be used for a single method invocation. The examples and documentation in the PR are generic, and show how to create an entirely new `RetrySettings` instance to supply via `withRetrySettings`, but there is API specific documentation that ggj could generate that would greatly help users. We should generate a single, client-level example of configuring `RetrySettings` via the `ApiCallContext` in the GAPIC layer that utilizes that GAPIC's `StubSettings` class as a base `RetrySettings` to modify. An example usage is as follows:

```java
LanguageServiceClient client = LanguageServiceClient.create();

// In order to adjust the RetrySettings on a per-call basis, modify the default settings
// and supply it via the ApiCallContext.
RetrySettings retrySettings =
client
.getSettings()
.analyzeSentimentSettings()
.getRetrySettings()
.toBuilder()
.setTotalTimeout(Duration.ofSeconds(10L))
.builder();

ApiCallContext callContext = GrpcCallContext.createDefault().withRetrySettings(retrySettings);

client.analyzeSentimentCallable().call(request, callContext);
```

_Note: The spacing is probably off there._

A similar existing example in [`LanguageServiceStubSettings`](https://github.com/googleapis/java-language/blob/master/google-cloud-language/src/main/java/com/google/cloud/language/v1/stub/LanguageServiceStubSettings.java#L70-L85).

I'm thinking it would be generated in this [client-level doc comment](https://github.com/googleapis/java-language/blob/master/google-cloud-language/src/main/java/com/google/cloud/language/v1/LanguageServiceClient.java#L61-L91) block somewhere.

cc: @miraleung