libc: Add a man page caveat about the w modifier

In-tree Clang is unaware of the N2680 fixed-width modifier `w` and emits
a warning when used.

PR:		299631
Reviewed by:	imp
Differential Revision: https://reviews.freebsd.org/D60939

lib/libc/stdio/printf.3

@@ -392,6 +392,12 @@ conversion:
 .It Sy Modifier Ta Cm c Ta Cm s
 .It Cm l No (ell) Ta Vt wint_t Ta Vt "wchar_t *"
 .El
+.Pp
+Compiler support for the
+.Cm w
+fixed-width integer type modifier is limited, and compilers may produce a
+warning when used (depending on compiler version and language standard
+settings).
 .It
 A character that specifies the type of conversion to be applied.
 .El