docs(bigquery): Add javadoc description of timestamp() parameter. (#3604)

* docs(bigquery): Add javadoc description of timestamp() parameter.

* Fix formatting.

Author: whuffman36

google-cloud-bigquery/src/main/java/com/google/cloud/bigquery/QueryParameterValue.java

@@ -300,7 +300,12 @@ public static QueryParameterValue bytes(byte[] value) {
     return of(value, StandardSQLTypeName.BYTES);
   }
 
-  /** Creates a {@code QueryParameterValue} object with a type of TIMESTAMP. */
+  /**
+   * Creates a {@code QueryParameterValue} object with a type of TIMESTAMP.
+   *
+   * @param value Microseconds since epoch, e.g. 1733945416000000 corresponds to 2024-12-11
+   *     19:30:16.929Z
+   */
   public static QueryParameterValue timestamp(Long value) {
     return of(value, StandardSQLTypeName.TIMESTAMP);
   }