plperl _init settings

There appears to be some significant misunderstanding of what can be 
done effectively using the various *_init settings for plperl.

In particular, some people have got an expectation that modules loaded 
in plperl.on_init will thereby be available for use in trusted plperl.

I propose to add the following note to the docs:

    Preloading modules using plperl.on_init does not make them available
    for use by plperl. External perl modules can only be used in plperlu.

Comments?

cheers

andrew