[DRAFT, WIP] PSR-5: PHPDoc Standard

This PR represents the current state of affairs with the PHPDoc Standard. Until this description and subject is altered this PR is considered a Work in Progress and not ready for a final review.

All feedback is welcome, please direct this to the PHP-FIG mailing list to prevent noise in this PR. This PSR covers so many subjects that unless the discussions are separated by topic that it will probably be quite impossible to follow in this PR.

I would also request that people open a thread per issue they would like to discuss and not combine several issues in a single mail; this will help to keep noise-levels down in discussions and for me to loook back or filter important information.

TODO items are tracked in the issue tracker for this PSR: https://github.com/phpDocumentor/fig-standards/issues
This requirement is violated by the current annotation tags in Doctrine/Symfony. Why is this a MUST? It will make a lot of source code immediately out of compliance with PSR-5, and hinder adoption.

Also, there are other documentation systems that already have their own tags not described by this standard, and their tags are presumably not prefixed by namespaces or vendor names. So they would also immediately be out of compliance, and the projects using them would not be able to adopt PSR-5.

My recommendation would be to remove this standard or change it to a SHOULD, if part of your goal is to promote adoption. I do not think this is necessary.