Document some of our client classes

This is mostly aiming to give an overview rather than be anything
comprehensive (there's another 230+ undocumented classes to go :p), but
it's a start.

Mostly just an excuse for me to procrastinate working on the nasty bugs
though!

projects/common-api/src/client/java/dan200/computercraft/api/client/ComputerCraftAPIClient.java

@@ -9,7 +9,11 @@ import dan200.computercraft.api.turtle.ITurtleUpgrade;
 import dan200.computercraft.api.turtle.TurtleUpgradeSerialiser;
 import dan200.computercraft.impl.client.ComputerCraftAPIClientService;
 
-
+/**
+ * The public API for client-only code.
+ *
+ * @see dan200.computercraft.api.ComputerCraftAPI The main API
+ */
 public final class ComputerCraftAPIClient {
     private ComputerCraftAPIClient() {
     }