Add a comment for myNotNullInfos

[Cherry-picked 44e63a1]

Author: noti0na1

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -782,6 +782,7 @@ class Namer { typer: Typer =>
 
     protected def localContext(owner: Symbol): FreshContext = ctx.fresh.setOwner(owner).setTree(original)
 
+    /** Stores the latest NotNullInfos (updated by `setNotNullInfos`) */
     private var myNotNullInfos: List[NotNullInfo] | Null = null
 
     /** The context with which this completer was created */