bpf: Add comment about helper freeze

Put a comment after the bpf helper list in uapi bpf.h to prevent people
from trying to add new helpers there and direct them to kfuncs.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Levi Zim <rsworktech@outlook.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Acked-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/bpf/CAEf4BzZvQF+QQ=oip4vdz5A=9bd+OmN-CXk5YARYieaipK9s+A@mail.gmail.com/
Link: https://lore.kernel.org/bpf/20221231004213.h5fx3loccbs5hyzu@macbook-pro-6.dhcp.thefacebook.com/
Link: https://lore.kernel.org/bpf/20250204-bpf-helper-freeze-v1-1-46efd9ff20dc@outlook.com

include/uapi/linux/bpf.h

@@ -6019,7 +6019,10 @@ union bpf_attr {
 	FN(user_ringbuf_drain, 209, ##ctx)		\
 	FN(cgrp_storage_get, 210, ##ctx)		\
 	FN(cgrp_storage_delete, 211, ##ctx)		\
-	/* */
+	/* This helper list is effectively frozen. If you are trying to	\
+	 * add a new helper, you should add a kfunc instead which has	\
+	 * less stability guarantees. See Documentation/bpf/kfuncs.rst	\
+	 */
 
 /* backwards-compatibility macros for users of __BPF_FUNC_MAPPER that don't
  * know or care about integer value that is now passed as second argument