HowToReleaseLLVM: Update document to match the current release process
- 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:
Reviewed By: hans
Differential Revision: https://reviews.llvm.org/D93493