crypto: add a note about libolm only being used in tests

It seems that the fact that matrix-rust-sdk contains `olm-rs` in its
`Cargo.lock` sparked panic, so let's attempt to fend off future concerns by
adding a comment.

crates/matrix-sdk-crypto/Cargo.toml

@@ -81,7 +81,11 @@ futures-executor = { workspace = true }
 http = { workspace = true }
 indoc = "2.0.1"
 matrix-sdk-test = { workspace = true }
+
+# libolm is deprecated. We use it only in tests, to ensure that our
+# implementation of `PkEncryption` is compatible with that in libolm.
 olm-rs = { version = "2.2.0", features = ["serde"] }
+
 proptest = { version = "1.0.0", default-features = false, features = ["std"] }
 similar-asserts = "1.5.0"
 # required for async_test macro