Document the nuances in UserId and SessionId types. (#1616)

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/core/SessionId.kt

@@ -16,4 +16,7 @@
 
 package io.element.android.libraries.matrix.api.core
 
+/**
+ * The [UserId] of the currently logged in user.
+ */
 typealias SessionId = UserId

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/core/UserId.kt

@@ -19,6 +19,11 @@ package io.element.android.libraries.matrix.api.core
 import io.element.android.libraries.matrix.api.BuildConfig
 import java.io.Serializable
 
+/**
+ * A [String] holding a valid Matrix user ID.
+ *
+ * https://spec.matrix.org/v1.8/appendices/#user-identifiers
+ */
 @JvmInline
 value class UserId(val value: String) : Serializable {