eclipse#333 - javadoc update - address review comments

Signed-off-by: Emily Jiang <emijiang6@googlemail.com>

api/src/main/java/org/eclipse/microprofile/config/spi/ConfigSource.java

@@ -85,7 +85,7 @@ public interface ConfigSource {
 
     /**
      * Gets all property names known to this config source, without evaluating the values. 
-     * The returned property names may be a subset of the configurations in this config source, due to security or performance reasons.
+     * The returned property names may be a subset of the names of the total set of retrievable properties in this config source.
      *
      * For backwards compatibility, there is a default implementation that just returns the keys of {@code getProperties()}
      * slower ConfigSource implementations should replace this with a more performant implementation