towlower.3: Largely rewrite description of towlower() to be simpler and clearer

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man3/towlower.3

@@ -1,4 +1,5 @@
 .\" Copyright (c) Bruno Haible <haible@clisp.cons.org>
+.\" and Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
 .\"
 .\" %%%LICENSE_START(GPLv2+_DOC_ONEPARA)
 .\" This is free documentation; you can redistribute it and/or
@@ -30,26 +31,28 @@ function is the wide-character equivalent of the
 function.
 If
 .I wc
-is a wide character, it is converted to
+is an uppercase wide character,
-lowercase.
+and there exists a lowercase equivalent in the current locale,
-Characters which do not have case are returned unchanged.
+it returns the lowercase equivalent of
-If
+.IR wc .
-.IR wc
+In all other cases,
-is
-.BR WEOF ,
-.B WEOF
-is returned.
-.SH RETURN VALUE
-The
-.BR towlower ()
-function returns the lowercase equivalent of
-.IR wc ,
-or
-.BR WEOF
-if
 .I wc
-is
+is returned unchanged.
+
+The argument
+.I wc
+must be representable as a
+.IR wchar_t
+and be a valid character in the locale or the value
 .BR WEOF .
+.SH RETURN VALUE
+If
+.I wc
+was convertible to lowercase,
+.BR towlower ()
+returns its lowercase equivalent;
+otherwise it returns
+.IR wc .
 .SH ATTRIBUTES
 .SS Multithreading (see pthreads(7))
 The