manual page tweak for $'...'

src/cmd/ksh93/sh.1:
- Tweak: document that $'...' is processed as a C99 string rather
  than an ANSI C (C90) string, because \u is supported and was
  added in C99.

src/cmd/ksh93/sh.1

@@ -3039,7 +3039,7 @@ are quoted.
 A single quote cannot appear within the single quotes.
 A single quoted string preceded by an unquoted
 .B $
-is processed as an ANSI C string
+is processed as a C99 string
 except for the following:
 .RS
 .PD 0