[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
So the general description MAY support Markdown, but tag descriptions MUST? This is inconsistent. 

Also I do not think that you should force all documentation parsers to support Markdown, and in any case, the wording on the general description is more accurate "Any application parsing the Description is RECOMMENDED to support the Markdown mark-up language" (accurate) vs. what is here "The description... support Markdown". The description is not supporting anything. The software that parses it is supporting markdown or not supporting markdown.