changelog: Mention fixes to +s channel leaks

[progval]

https://github.com/matrix-org/matrix-appservice-irc/releases/tag/0.38.0 will need to be updated accordingly if this is accepted

[progval]

Signed-off-by: Valentin Lorentz \<progval plus matrixsignoff at progval dot net>

[Half-Shot]

I've modified this a bit so we combine the lines (as really it's the same bug with two causes), and toned the language a bit for users/administrators. Otherwise, looks good to me. Thanks for keeping us honest!

[mweinelt]

Thanks for clarifying that.

[progval]

Thanks!