spanner: add comments to the timeout/retry code sample #5310
Description
Add some comments to explain the parameters: https://github.com/googleapis/java-spanner/blob/HEAD/samples/snippets/src/main/java/com/example/spanner/CustomTimeoutAndRetrySettingsExample.java#L55-L65
Like what we have done in nodejs: https://github.com/googleapis/nodejs-spanner/blob/de6e0e5115a3d3da5bfe5f941cbf8fc6165b4b68/samples/dml.js#L655-L682