CO: Document Attachment class

[firstred]

Questions	Answers
Branch?	develop
Description?	Upgrading the PHPDoc comments of this class, using the latest PS standard. Also getting the Franglais out of the comments, getting rid of things my Symfony code sniffer nags about, etc. It might have some additional todo tags, depending on whether the code was comprehensible or not.
Type?	improvement
Category?	CO
BC breaks?	no
Deprecations?	no
Fixed ticket?	N/A
How to test?	N/A

[mickaelandrieu]

LGTM :) thanks