Revised Javadoc of `RestoreSystemProperties`.

Add some additional information and simplified the example. Hopefully it
makes it easier to understand the rule.
RestoreSystemProperties restores all properties.

Simplifies usage of the rule. Users just need to paste the snippet.

    @Rule
    public final TestRule restoreSystemProperties
            = new RestoreSystemProperties();

and every property is restored.
Deprecated clearProperty(String).

This method is no longer needed because RestoreSystemProperties
restores all properties. Now the user can use System.clearProperty
in its test.