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