GDP: link policy

[Graham Percival]

I've updated the policy on links in GDP.  Sorry for the delay in this.

`rough work' doc writers: don't worry about congregating all the links 
in the @seealso; leave those details for the formatters.


* You _must_ include a @seealso.  The order of items inside the
  @seealso section is

    Music glossary: @rglos{foo}, @rglos{bar}.

    Learning Manual: @rlearning{baz}, @rlearning{foozle}

    Notation Reference: @ruser{faazle}, @ruser{boo}.

    Snippets: @lsrdir{section}, @lsr{specific/example-name.ly}

    Program reference: @internalsref{fazzle}, @internalsref{booar}.

  ("Snippets" is REQUIRED; the others are optional)

...

* References must occur at the end of a sentence, for more
  information see @ref{the texinfo manual}.  Ideally this should
  also be the final sentence of a paragraph, but this is not
  required.  Any link in a doc section must be duplicated in the
  @seealso section at the bottom.



Cheers,
- Graham