[V3] Enforce use of redbot.core.commands (#1971)

* enforce commands as ours

* clearer user feedback

* No more 'one more tweak' commits without verifying anyway

* more detailed error with docs link + docs update

Resolves #1972.

redbot/core/bot.py

@@ -291,6 +291,22 @@ class RedBase(BotBase, RPCMixin):
             if pkg_name.startswith("redbot.cogs."):
                 del sys.modules["redbot.cogs"].__dict__[name]
 
+    def add_cog(self, cog):
+        for attr in dir(cog):
+            _attr = getattr(cog, attr)
+            if isinstance(_attr, discord.ext.commands.Command) and not isinstance(
+                _attr, commands.Command
+            ):
+                raise RuntimeError(
+                    f"The {cog.__class__.__name__} cog in the {cog.__module__} package,"
+                    " is not using Red's command module, and cannot be added. "
+                    "If this is your cog, please use from `redbot.core import commands`"
+                    "in place of `from discord.ext import commands`. For more details on "
+                    "this requirement, see this page: "
+                    "http://red-discordbot.readthedocs.io/en/v3-develop/framework_commands.html"
+                )
+        super().add_cog(cog)
+
 
 class Red(RedBase, discord.AutoShardedClient):
     """