Problem/Motivation
Drupal\Core\Render\AttachmentsInterface is basically as bad as the legendary ContextInterface was "interface for context".
It doesn't document what is in the $attachments array neither does it elaborate on the difference between set() and add().
Steps to reproduce
Open AttachmentsInterface . Try to figure it out what it does. Try to figure out the difference between set and add.
Proposed resolution
1) Move AttachmentsResponseProcessorInterface::processAttachments() doxygen into the AttachmentsInterface class doxygen .
2) refer to the class doxygen in every method doxygen and processAttachments() too.
3) Explain set vs add.
Remaining tasks
Review
User interface changes
Not relevant.
API changes
Not relevant.
Data model changes
Not relevant.
Release notes snippet
Not relevant.
