Document potential breaking changes in multithreaded mode.

[AR-May]

Here is the (incomplete) list of potential breaking changes in multithreaded mode. These changes may cause differences in behavior compared to multiprocess execution:

1. Environment variables became case-sensitive on unix. Relevant links: Environment variable keys should be case sensitive on unix #12858, plumbing taskenvironment through execution #12842.
2. Absolute paths verification uses Path.IsPathFullyQualified instead of Path.IsPathRooted. Relevant links: Path.IsPathRooted vs Path.IsPathFullyQualified #12849, plumbing taskenvironment through execution #12842.
3. Builds that include non-multithreadable tasks using custom task factories will fail because the task host does not support custom task factories.