d: remove the default prefix for SymbolKind's enum elements

This is not needed in D, since the kinds are "scoped".

* data/skeletons/d.m4: Remove the default prefix.
* doc/bison.text: Document it.

data/skeletons/d.m4

@@ -197,8 +197,6 @@ b4_symbol_foreach([b4_token_enum])dnl
 ## Symbol kinds.  ##
 ## -------------- ##
 
-b4_percent_define_default([[api.symbol.prefix]], [[S_]])
-
 # b4_symbol_kind(NUM)
 # -------------------
 m4_define([b4_symbol_kind],

doc/bison.texi

@@ -6420,7 +6420,7 @@ Any non empty string.  Must be a valid identifier in the target language
 (typically a non empty sequence of letters, underscores, and ---not at the
 beginning--- digits).
 
-The empty prefix is invalid:
+The empty prefix is (generally) invalid:
 @itemize
 @item
 in C it would create collision with the @code{YYERROR} macro, and
@@ -6436,7 +6436,8 @@ with other members of the @code{SymbolKind} class.
 
 
 @item Default Value:
-@code{YYSYMBOL_} in C.  @code{S_} in C++, D and Java.
+@code{YYSYMBOL_} in C.  @code{S_} in C++ and Java.  The default prefix is
+removed from D.
 @item History:
 introduced in Bison 3.6.
 @end itemize