doc: clarify the contract of get(String)

src/main/java/spoon/reflect/factory/TypeFactory.java

@@ -402,9 +402,13 @@ public <T> CtTypeReference<T> createReference(String qualifiedName) {
 	}
 
 	/**
-	 * Gets a created type from its qualified name.
+	 * Gets a created type from its qualified name if source in the source classpath. 
 	 *
-	 * @return a found type or null if does not exist
+	 * `TypeFactory#get(String)` returns null if the class is not in the source classpath (even if it is in the binary classpath).
+	 * `TypeFactory#get(Class)` returns null if the class is neither in the source classpath nor in the binary classpath, 
+	 * and returns a [shadow class](http://spoon.gforge.inria.fr/reflection.html) if it is only in the binary classpath. 
+	 * Note that a shadow class has an empty method body, if you need a shadow class with method body, see [spoon-decompiler](https://github.com/INRIA/spoon/tree/master/spoon-decompiler))
+	 * @return a type if source in the source classpath or null if does not exist
 	 */
 	@SuppressWarnings("unchecked")
 	public <T> CtType<T> get(final String qualifiedName) {