api.header.include: document it, and fix its default value

While defining api.header.include worked as expected, its default
value was incorrectly defined.  As a result, by default, the generated
parsers still duplicated the content of the generated header instead
of including it.

* data/skeletons/yacc.c (api.header.include): Fix its default value.
* tests/output.at: Check it.
* doc/bison.texi (%define Summary): Document api.header.include.
While at it, move the definition of api.namespace at the proper
place.

TODO

@@ -15,16 +15,11 @@ list_get_end (gl_list_t list)
 ** Bistromathic
 - Hitting tab on a line with a syntax error is ugly
 
-- Be robust to existing ~/.inputrc
-
 - How about not evaluating incomplete lines when the text is not finished
   (as shells do).
 
 - Caret diagnostics
 
-** Doc
-*** api.header.include
-
 ** Questions
 *** Java
 - Should i18n be part of the Lexer?  Currently it's a static method of
@@ -54,9 +49,6 @@ enough.
 *** calc.at
 Stop hard-coding "Calc".  Adjust local.at (look for FIXME).
 
-** Counter example generation
-See https://github.com/akimd/bison/pull/15.
-
 ** Clean up
 Rename endtoken as eoftoken.