docs: Add missing javadoc (#121)

This shows up as a warning during the build.
Though the method is deprecated, it's worth documenting it.

See: jenkinsci/gradle-jpi-plugin#241 (comment)

Co-authored-by: Rahul Somasunderam <rahulsom@noreply.github.com>

Author: rahulsom

src/main/java/hudson/util/VersionNumber.java

@@ -545,6 +545,11 @@ public boolean isNewerThanOrEqualTo(VersionNumber rhs) {
     }
 
     /**
+     * Returns a digit (numeric component) by its position. Once a non-numeric component is found all remaining components
+     * are also considered non-numeric by this method.
+     *
+     * @param idx Digit position we want to retrieve starting by 0.
+     * @return The digit or -1 in case the position does not correspond with a digit.
      * @deprecated see {@link #getDigitAt(int)}
      */
     @Deprecated