Add docs for different metadata versions and proto files

Review: https://jetbrains.team/p/kt/reviews/8401
In scope of KT-55082

compiler/ir/serialization.common/src/KotlinIr.proto

@@ -1,6 +1,8 @@
 syntax = "proto2";
 package org.jetbrains.kotlin.backend.common.serialization.proto;
 
+// Changes to this proto are tracked by KotlinAbiVersion version. See KotlinAbiVersion KDoc.
+
 option java_multiple_files = true;
 option java_outer_classname = "KotlinIr";
 option optimize_for = LITE_RUNTIME;