document -W as per charles patch message

Notes:
Fri Mar 8 15:14:57 EST 2013 geoff
    already done by forsyth.


Reference: /n/sources/patch/sorry/cc-warn2errs-man
Date: Wed Mar  6 16:33:37 CET 2013
Signed-off-by: quanstro@quanstro.net
Reviewed-by: geoff

--- /sys/man/1/8c	Wed Mar  6 16:33:17 2013
+++ /sys/man/1/8c	Wed Mar  6 16:33:15 2013
@@ -109,6 +109,12 @@
 .B -w
 Print warning messages about unused variables, etc.
 .TP
+.B -W
+Similar to 
+.BR -w ,
+but makes warnings into errors,
+so that large builds that should be warning-free will stop in their tracks.
+.TP
 .B -B
 Accept functions without a new-style
 ANSI C function prototype.