docs(docstrings): add docstrings to public method interfaces (#25)

- Resolves #18

Reviewed-on: #25

src/main/java/com/ddf/vodsystem/exceptions/ClipMediaUnavailable.java

@@ -12,6 +12,11 @@ public class ClipMediaUnavailable extends RuntimeException {
     this.clipId = clipId;
   }
 
+  /**
+   * Returns the ID of the clip whose media was unavailable.
+   *
+   * @return the affected clip's ID
+   */
   public Long getClipId() {
     return clipId;
   }