doc: change error message testing policy

Dynamic error messages often contain important information that
depends on the context, e.g., which argument caused an
ERR_INVALID_ARG_TYPE, which type was expected, and which type was
received. I don't think that internal breakage when changing a dynamic
error message should prevent us from testing such properties properly.

PR-URL: #31421
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/guides/using-internal-errors.md

@@ -111,8 +111,8 @@ assert.strictEqual(
 ```
 
 In addition, there should also be tests which validate the use of the
-error based on where it is used in the codebase.  For these tests, except in
-special cases, they should only validate that the expected code is received
+error based on where it is used in the codebase.  If the error message is
+static, these tests should only validate that the expected code is received
 and NOT validate the message.  This will reduce the amount of test change
 required when the message for an error changes.