Spanner: Added extra documentation to the DatabaseClient.singleUse methods (#4721)

* added extra documentation to the singleUse methods #4212

* changed formatting manually and re-run mvn formatter

Author: olavloite

google-cloud-clients/google-cloud-spanner/src/main/java/com/google/cloud/spanner/DatabaseClient.java

@@ -85,7 +85,9 @@ public interface DatabaseClient {
 
   /**
    * Returns a context in which a single read can be performed using {@link TimestampBound#strong()}
-   * concurrency.
+   * concurrency. This method will return a {@link ReadContext} that will not return the read
+   * timestamp that was used by Cloud Spanner. If you want to be able to access the read timestamp,
+   * you should use the method {@link #singleUseReadOnlyTransaction()}.
    *
    * <p>Example of single use.
    *
@@ -100,17 +102,18 @@ public interface DatabaseClient {
   ReadContext singleUse();
 
   /**
-   * Returns a context in which a single read can be performed at the given timestamp bound.
+   * Returns a context in which a single read can be performed at the given timestamp bound. This
+   * method will return a {@link ReadContext} that will not return the read timestamp that was used
+   * by Cloud Spanner. If you want to be able to access the read timestamp, you should use the
+   * method {@link #singleUseReadOnlyTransaction()}.
    *
    * <p>Example of single use with timestamp bound.
    *
    * <pre>{@code
    * long singerId = my_singer_id;
    * String column = "FirstName";
-   * Struct row =
-   *     dbClient
-   *         .singleUse(TimestampBound.ofMaxStaleness(10, TimeUnit.SECONDS))
-   *         .readRow("Singers", Key.of(singerId), Collections.singleton(column));
+   * Struct row = dbClient.singleUse(TimestampBound.ofMaxStaleness(10, TimeUnit.SECONDS))
+   *     .readRow("Singers", Key.of(singerId), Collections.singleton(column));
    * String firstName = row.getString(column);
    * }</pre>
    *