Rephrase explanation

Author: som-snytt

Diff for: compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2520,8 +2520,8 @@ class ExtensionHasDefault(method: Symbol)(using Context)
   def msg(using Context) =
     i"""Extension method ${hl(method.name.toString)} should not have a default argument for its receiver."""
   def explain(using Context) =
-    i"""Although extensions are ordinary methods, when they are invoked as a selection,
-       |the receiver cannot be omitted. A default argument for that parameter would never be used.
+    i"""The receiver cannot be omitted when an extension method is invoked as a selection.
+       |A default argument for that parameter would never be used in that case.
        |An extension method can be invoked as a regular method, but if that is the intended usage,
        |it should not be defined as an extension."""
 

Diff for: tests/warn/i12460.check

@@ -5,8 +5,8 @@
   |---------------------------------------------------------------------------------------------------------------------
   | Explanation (enabled by `-explain`)
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-  | Although extensions are ordinary methods, when they are invoked as a selection,
-  | the receiver cannot be omitted. A default argument for that parameter would never be used.
+  | The receiver cannot be omitted when an extension method is invoked as a selection.
+  | A default argument for that parameter would never be used in that case.
   | An extension method can be invoked as a regular method, but if that is the intended usage,
   | it should not be defined as an extension.
    ---------------------------------------------------------------------------------------------------------------------
@@ -17,8 +17,8 @@
   |---------------------------------------------------------------------------------------------------------------------
   | Explanation (enabled by `-explain`)
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-  | Although extensions are ordinary methods, when they are invoked as a selection,
-  | the receiver cannot be omitted. A default argument for that parameter would never be used.
+  | The receiver cannot be omitted when an extension method is invoked as a selection.
+  | A default argument for that parameter would never be used in that case.
   | An extension method can be invoked as a regular method, but if that is the intended usage,
   | it should not be defined as an extension.
    ---------------------------------------------------------------------------------------------------------------------