doco keyword primitives

[Kevin Ryde]

        * scheme-data.texi (Keyword Primitives): Add examples to
        make-keyword-from-dash-symbol and keyword-dash-symbol.  Add
        scm_c_make_keyword.

Should it be documented that scm_c_make_keyword doesn't use the given
string once it returns?  I think that's how it works now, don't know
if this might want to change in the future so it could use a literal
string in rodata directly, or whatever.


 - Scheme Procedure: make-keyword-from-dash-symbol symbol
 - C Function: scm_make_keyword_from_dash_symbol (symbol)
     Make a keyword object from a SYMBOL that starts with a dash.  For
     example,

          (make-keyword-from-dash-symbol '-foo)
          => #:foo

 - Scheme Procedure: keyword-dash-symbol keyword
 - C Function: scm_keyword_dash_symbol (keyword)
     Return the dash symbol for KEYWORD.  This is the inverse of
     `make-keyword-from-dash-symbol'.  For example,

          (keyword-dash-symbol #:foo)
          => -foo

 - C Function: SCM scm_c_make_keyword (char *STR)
     Make a keyword object from a string.  For example,

          scm_c_make_keyword ("foo")
          => #:foo

     The new keyword doesn't use the given C string, so it can be freed
     or rewritten as desired.