isalpha.3: Note circumstances where 'c' must be cast to 'unsigned char'

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

man3/isalpha.3

@@ -312,6 +312,38 @@ and
 .BR isascii_l ()
 is a GNU extension.
 .SH NOTES
+The standards require that the argument
+.I c
+for these functions is either
+.B EOF
+or a value that is representable in the type
+.IR "unsigned char" .
+If the argument
+.I c
+is of type
+.IR char ,
+it must be cast to
+.IR "unsigned char" ,
+as in the following example:
+
+.nf
+.in +4n
+char c;
+\&...
+res = toupper((unsigned char) c);
+.in
+.fi
+
+This is necessary because
+.I char
+may be the equivalent
+.IR "signed char" ,
+in which case a byte where the top bit is set would be sign extended when
+converting to
+.IR int ,
+yielding a value that is outside the range of
+.IR "unsigned char" .
+
 The details of what characters belong to which class depend on the
 locale.
 For example,