HowToReleaseLLVM: Update document to match the current release process
Change Summary:
* Clarify that release manager can commit without code owner approval
(but are still highly encouraged to get approval).
* Clarify that there is no official release criteria.
* Document what types of changes are allowed in each release phase.
This is update is based on the RFC submitted here:
http://lists.llvm.org/pipermail/llvm-dev/2020-May/141730.html
Reviewed By: hans
Differential Revision: https://reviews.llvm.org/D93493