Update doc pages

New doc page on mutability, split off from "Separation Checking".

Author: odersky

compiler/src/dotty/tools/dotc/cc/CaptureOps.scala

@@ -352,8 +352,8 @@ extension (tp: Type)
     case _ =>
       false
 
-  /** Is this a type extending `Mutable` that has update methods
-   *  or non-private mutable fields?
+  /** Is this a type extending `Mutable` that has non-private update methods
+   *  or mutable fields?
    */
   def isMutableType(using Context): Boolean =
     tp.derivesFrom(defn.Caps_Mutable)

compiler/src/dotty/tools/dotc/cc/CaptureSet.scala

@@ -640,9 +640,9 @@ object CaptureSet:
       else ""
 
   private def capImpliedByCapability(parent: Type)(using Context): Capability =
-    if parent.derivesFromExclusive then GlobalCap.readOnly else GlobalCap
+    if parent.derivesFromMutable then GlobalCap.readOnly else GlobalCap
 
-  /* The same as {cap.rd} but generated implicitly for references of Capability subtypes.
+  /* The same as {cap} but generated implicitly for references of Capability subtypes.
    */
   class CSImpliedByCapability(parent: Type)(using @constructorOnly ctx: Context)
   extends Const(SimpleIdentitySet(capImpliedByCapability(parent)))