WSL disclaimer in Getting Started/Prerequisites (pantsbuild#18000)

A short disclaimer was added to inform new Windows users that their projects must live within the Linux virtual machine for Pants to work correctly. 

Feedback welcome! In particular, I want to confirm that using the `wsl` prefix in a Windows shell has the same limitations. I will check myself in a few days if no one knows already.

Related to discussion in pantsbuild#17986.

docs/markdown/Getting Started/getting-started/prerequisites.md

@@ -68,3 +68,7 @@ System-specific notes
 > - Install a recent Linux distribution under WSL 2 (we have tested with Ubuntu 20.04 LTS).
 > - Run `sudo apt install unzip python3-dev python3-distutils python3-venv gcc` in the distribution.
 > - You can then run Pants commands in a Linux shell, or in a Windows shell by prefixing with `wsl `.
+> 
+> Projects using Pants must be contained within the Linux virtual machine:
+> 
+> - Navigating a Linux shell to a Windows directory via the `/mnt` directory, or using the `wsl` prefix with a Windows shell in a Windows directory, and executing Pants may result in unexpected behavior.