Complete documentation of syntax flags by adding `c'

* doc/lispref/syntax.texi ("Syntax Table Internals"): Add entry for `c'
as `(1 lsh 23)'.

doc/lispref/syntax.texi

@@ -1018,7 +1018,7 @@ corresponds to each syntax flag.
 @item
 @samp{3} @tab @code{(lsh 1 18)} @tab @samp{n} @tab @code{(lsh 1 22)}
 @item
-@samp{4} @tab @code{(lsh 1 19)}
+@samp{4} @tab @code{(lsh 1 19)} @tab @samp{c} @tab @code{(lsh 1 23)}
 @end multitable
 
 @defun string-to-syntax desc