Clarify behavior of tab completing in command blocks

By: Pokechu22 <Pokechu022@gmail.com>

paper-api/src/main/java/org/bukkit/command/TabCompleter.java

@@ -10,7 +10,9 @@ public interface TabCompleter {
     /**
      * Requests a list of possible completions for a command argument.
      *
-     * @param sender Source of the command
+     * @param sender Source of the command.  For players tab-completing a
+     *     command inside of a command block, this will be the player, not
+     *     the command block. 
      * @param command Command which was executed
      * @param alias The alias used
      * @param args The arguments passed to the command, including final