diag.t: subDivide exception list

Some of the entries in this list of diagnostics are ones that people
just haven't gotten around to documenting.

Patches welcome!

But there are, in my opinion, a very few are ones that don't ever need
to be documented because the text of the diganostic is sufficiently
explanatory in and of itself.

This commit revises the comments explaining the exception list, and
moves the ones I think are sufficiently self-explanatory to the front.

t/porting/diag.t

@@ -519,19 +519,34 @@ sub check_message {
     return $ret;
 }
 
-# Lists all missing things as of the inauguration of this script, so we
-# don't have to go from "meh" to perfect all at once.
-# 
-# PLEASE DO NOT ADD TO THIS LIST.  Instead, write an entry in
-# pod/perldiag.pod for your new (warning|error).  Nevertheless,
-# listing exceptions here when this script is not smart enough
-# to recognize the messages is not so bad, as long as there are
-# entries in perldiag.
-
-# Entries after __CATEGORIES__ are those that are in perldiag but fail the
-# severity/category test.
+# The DATA section includes two types of entries, in two sets separated by a
+# blank line.
+#
+# The first set are entries whose text we think is fully explanatory and don't
+# need further elaboration in perldiag.  This set should consist of very few
+# entries.
+#
+# The second set, after the blank line, consists of TODO entries.  (There are
+# actually two subsets described below.)  This list should basically be just
+# those entries that otherwise would have generated an error upon inauguration
+# of this program, so we didn't have to go from "meh" to perfect all at once.
+# The only valid reason we can think of to add to the list is for cases where
+# this program is not smart enough to recognize the message is something that
+# actually is in perldiag.  Otherwise, DO NOT ADD TO THIS LIST.  Instead,
+# write an entry in pod/perldiag.pod for your new (warning|error).
+#
+# This second set has a subcategory, after the line marked __CATEGORIES__ .
+# These are entries that are in perldiag but fail the severity/category test.
 
 __DATA__
+Function "%s" not implemented in this version of perl.
+QUITing...
+Recompile perl with -DDEBUGGING to use -D switch (did you mean -d ?)
+System V IPC is not implemented on this machine
+Terminating on signal SIG%s(%d)
+This version of OS/2 does not support %s.%s
+Usage: %s
+
 Cannot apply "%s" in non-PerlIO perl
 Can't find DLL name for the module `%s' by the handle %d, rc=%u=%x
 Can't find string terminator %c%s%c anywhere before EOF
@@ -574,7 +589,6 @@ fork() not implemented!
 YOU HAVEN'T DISABLED SET-ID SCRIPTS IN THE KERNEL YET! FIX YOUR KERNEL, PUT A C WRAPPER AROUND THIS SCRIPT, OR USE -u AND UNDUMP!
 free %s
 Free to wrong pool %p not %p
-Function "%s" not implemented in this version of perl.
 get %s %p %p %p
 gethostent not implemented!
 getnetbyaddr not implemented!
@@ -616,8 +630,6 @@ panic queryaddr
 Parse error
 POSIX syntax [%c %c] is reserved for future extensions in regex; marked by <-- HERE in m/%s/
 ptr wrong %p != %p fl=%x nl=%p e=%p for %d
-QUITing...
-Recompile perl with -DDEBUGGING to use -D switch (did you mean -d ?)
 recursion detected in %s
 Reversed %c= operator
 %s: Can't parse EXE/DLL name: '%s'
@@ -643,10 +655,7 @@ Starting Full Screen process with flag=%d, mytype=%d
 Starting PM process with flag=%d, mytype=%d
 switching effective gid is not implemented
 switching effective uid is not implemented
-System V IPC is not implemented on this machine
-Terminating on signal SIG%s(%d)
 This perl was compiled without taint support. Cowardly refusing to run with -t or -T flags
-This version of OS/2 does not support %s.%s
 Too deeply nested ()-groups in %s
 Too many args on %s line of "%s"
 U0 mode on a byte string
@@ -656,7 +665,6 @@ Unexpected program mode %d when morphing back from PM
 Unrecognized character %s; marked by <-- HERE after %s<-- HERE near column %d
 Unstable directory path, current directory changed unexpectedly
 Unterminated compressed integer in unpack
-Usage: %s
 utf8 "\x%X" does not map to Unicode
 Value of logical "%s" too long. Truncating to %i bytes
 waitpid: process %x is not a child of process %x