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:

• Environment variables became case-sensitive on unix. Relevant links: Environment variable keys should be case sensitive on unix #12858, plumbing taskenvironment through execution #12842.
• Absolute paths verification uses Path.IsPathFullyQualified instead of Path.IsPathRooted. Relevant links: Path.IsPathRooted vs Path.IsPathFullyQualified #12849, plumbing taskenvironment through execution #12842.
• Document that in MT mode Custom Task Factories are not supported #13026