Add brief documentation of overlay classes

It could be expanded in future, but it's better than what was there
before

Author: Owen Jones

jbmc/src/java_bytecode/java_class_loader.cpp

@@ -69,8 +69,16 @@ java_class_loadert::parse_tree_with_overlayst &java_class_loadert::operator()(
 }
 
 /// Check if class is an overlay class by searching for `ID_overlay_class` in
-/// its list of annotations. TODO(nathan) give a short explanation about what
-/// overlay classes are.
+/// its list of annotations.
+///
+/// An overlay class is a class with the annotation @OverlayClassImplementation.
+/// They serve the following purpose. When the JVM searches the classpath for a
+/// particular class, it uses the first one it finds that matches, and ignores
+/// any other matching ones. JBMC, however, will take account of other matching
+/// ones which are overlay classes. The overlay classes can add or
+/// replace parts of the first matching class, e.g. adding a new method or
+/// changing the definition of a method.
+///
 /// \param c: a `classt` object from a java byte code parse tree
 /// \return true if parsed class is an overlay class, else false
 static bool is_overlay_class(const java_bytecode_parse_treet::classt &c)