Update documentation about the incremental collector support

Update documentation about the incremental collector support
(a cherry-pick of commit 5d4fd4de from 'master')

* doc/README.darwin (Important Usage Notes): Remove outdated note about
flaky incremental mode.
* doc/README.darwin (Implementation Information): Remove information
about old incremental collector implementation.
* doc/README.win32 (Watcom compiler): Refine information about the
incremental mode support.
* doc/scale.md (Options for enhanced scalability): Refine when the
dedicated marker threads are created.