update comment (#276)

- [x] I ran `make setup && make` to update the generated code after
editing a `.atd` file (TODO: have a CI check)
- [x] I made sure we're still backward compatible with old versions of
the CLI.
For example, the Semgrep backend need to still be able to *consume* data
generated
	  by Semgrep 1.17.0.
See
https://atd.readthedocs.io/en/latest/atdgen-tutorial.html#smooth-protocol-upgrades

Author: ihji

rule_schema_v2.atd

@@ -410,6 +410,8 @@ type metavariable_cond = {
   ?type_ <json name="type">: string option;
   ?types: string list option;
 
+  (* for semgrep-internal-metavariable-name, currently only support
+     "django-view" kind *)
   ?kind: string option;
 
   (* this covers regex:/pattern:, but also all:/any: with optional where: