Freeze note on `SessionInfo`. (#6818)

* Add note about frozen `SessionInfo`.

* Better comment.

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Co-authored-by: eskimor <eskimor@no-such-url.com>

polkadot/primitives/src/v2/mod.rs

@@ -1603,6 +1603,9 @@ pub fn supermajority_threshold(n: usize) -> usize {
 }
 
 /// Information about validator sets of a session.
+///
+/// NOTE: `SessionInfo` is frozen. Do not include new fields, consider creating a separate runtime
+/// API. Reasoning and further outlook [here](https://github.com/paritytech/polkadot/issues/6586).
 #[derive(Clone, Encode, Decode, RuntimeDebug, TypeInfo)]
 #[cfg_attr(feature = "std", derive(PartialEq))]
 pub struct SessionInfo {