Apply suggestions from code review

Co-authored-by: Ondřej Lhoták <olhotak@uwaterloo.ca>

Author: noti0na1

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

@@ -52,8 +52,11 @@ object Nullables:
     val hiTree = if(hiTpe eq hi.typeOpt) hi else TypeTree(hiTpe)
     TypeBoundsTree(lo, hiTree, alias)
 
-  /** A set of val or var references that are known to be not null,
-   *  plus a set of variable references that are once assigned to null.
+  /** A set of val or var references that are known to be not null
+  * after the tree finishes executing normally (non-exceptionally),
+   *  plus a set of variable references that are ever assigned to null,
+   *  and may therefore be null if execution of the tree is interrupted
+   *  by an exception.
    */
   case class NotNullInfo(asserted: Set[TermRef], retracted: Set[TermRef]):
     def isEmpty = this eq NotNullInfo.empty

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

@@ -2360,7 +2360,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     val cases2 = cases2x.asInstanceOf[List[CaseDef]]
 
     // It is possible to have non-exhaustive cases, and some exceptions are thrown and not caught.
-    // Therefore, the code in the finallizer and after the try block can only rely on the retracted
+    // Therefore, the code in the finalizer and after the try block can only rely on the retracted
     // info from the cases' body.
     if cases2.nonEmpty then
       nnInfo = nnInfo.seq(cases2.map(_.notNullInfo.retractedInfo).reduce(_.alt(_)))

tests/explicit-nulls/neg/i21380c.scala

@@ -33,7 +33,7 @@ def test4: Int =
     case _ => x = ""
   x.length // error
   // Although the catch block here is exhaustive, it is possible to have non-exhaustive cases,
-  // and some exceptions are thrown and not caught. Therefore, the code in the finallizer and
+  // and some exceptions are thrown and not caught. Therefore, the code in the finalizer and
   // after the try block can only rely on the retracted info from the cases' body.
 
 def test5: Int =