[Documentation] Document known caveats to diagnostic tools

[sywhang]

The tools can cause certain behaviors that are not favorable in prod environments. Document these so that users can more easily adopt their tools in prod environment and understand the implications of running these tools in such environments.

• dotnet-gcdump: Triggers Gen 2 GC
• dotnet-trace: Rundown can take a long time.
• dotnet-stack: Rundown can take a long time.
• Tools targeting x86 need to be started using the x86 host.
• Tools need to share a TMPDIR environment variable value with the target application.
• Tools need run as same user as the target application to access diagnostic pipes

[sywhang]

Leaving this issue open to make sure dotnet-stack docs go in with all these checked.