Resolve or eliminate Environment-related TODOs

Issue: SPR-8031, SPR-7508

org.springframework.core/src/main/java/org/springframework/core/env/EnumerablePropertySource.java

@@ -19,7 +19,23 @@ package org.springframework.core.env;
 import org.springframework.util.Assert;
 
 /**
- * TODO SPR-7508: document
+ * A {@link PropertySource} implementation capable of interrogating its
+ * underlying source object to enumerate all possible property key/value
+ * pairs. Exposes the {@link #getPropertyNames()} method to allow callers
+ * to introspect available properties without having to access the underlying
+ * source object. This also facilitates a more efficient implementation of
+ * {@link #containsProperty(String)}, in that it can call {@link #getPropertyNames()}
+ * and iterate through the returned array rather than attempting a call to
+ * {@link #getProperty(String)} which may be more expensive. Implementations may
+ * consider caching the result of {@link #getPropertyNames()} to fully exploit this
+ * performance opportunity.
+ *
+ * Most framework-provided {@code PropertySource} implementations are enumerable;
+ * a counter-example would be {@code JndiPropertySource} where, due to the
+ * nature of JNDI it is not possible to determine all possible property names at
+ * any given time; rather it is only possible to try to access a property
+ * (via {@link #getProperty(String)}) in order to evaluate whether it is present
+ * or not.
  *
  * @author Chris Beams
  * @since 3.1

org.springframework.core/src/main/java/org/springframework/core/env/PropertySource.java

@@ -214,9 +214,11 @@ public abstract class PropertySource<T> {
 			super(name, new Object());
 		}
 
+		/**
+		 * Always return {@code null}.
+		 */
 		@Override
 		public String getProperty(String key) {
-			// TODO SPR-7408: logging
 			return null;
 		}
 	}