allow porting/diag.t to detect messages from XSUBs in class.c

and briefly document the undocumented messages it found.

Note that the "Odd number of arguments" message isn't suppressible,
I do not know if that is by design:

  $ ./perl -Ilib -Mfeature=class -e 'no warnings; class C { field $x:param; } C->new("x")'
  Odd number of arguments passed to "C" constructor at -e line 1.

Discovered when porting/diag.t didn't warn about my new message
in the fix for #22159

pod/perldiag.pod

@@ -4741,6 +4741,14 @@ regardless of what name the caller used.
 (W overload) The call to overload::constant contained an odd number of
 arguments.  The arguments should come in pairs.
 
+=item Odd number of arguments passed to "%s" constructor
+
+(S) The constructor for the given class was called with an odd
+number of arguments.
+
+Class constructors accept named arguments as a list and expect pairs
+of name followed by value as if you were initialising a hash.
+
 =item Odd number of elements in anonymous hash
 
 (W misc) You specified an odd number of elements to initialize a hash,
@@ -7320,6 +7328,11 @@ whereabouts in the regular expression the escape was discovered.
 (F) You attempted to add a named attribute to a C<field> definition, but
 perl does not recognise the name of the requested attribute.
 
+=item Unrecognised parameters for "%s" constructor: %s
+
+(F) You called new on a class but supplied a parameter name that
+didn't match a class field name.
+
 =item Unrecognized signal name "%s"
 
 (F) You specified a signal name to the kill() function that was not

t/porting/diag.t

@@ -243,6 +243,7 @@ my $specialformats_re = qr/%$format_modifiers"\s*($specialformats)(\s*(?:"|\z))?
 # We skip the bodies of most XS functions, but not within these files
 my @include_xs_files = (
   "builtin.c",
+  "class.c",
   "universal.c",
 );