Update DEVGUIDE.md

[smoothdeveloper]

Add a section about updating FCS surface area baselines specifically.

Credit to @auduchinok

[vzarytovskii]

This is not needed for surface area, only test baselines (unless something changed lately, and if it does, it needs to be reverted to not require that flag).

[nojaf]

You sure?

fsharp/tests/FSharp.Compiler.Service.Tests/SurfaceArea.fs

Lines 9 to 18 in 4c60c63

	type SurfaceAreaTest() =
	// This relies on a set of baselines to update the baseline set an environment variable before running the tests, then on failure the baselines will be updated
	// Handled by SurfaceArea.verify
	//
	// CMD:
	// set TEST_UPDATE_BSL=1
	// PowerShell:
	// $env:TEST_UPDATE_BSL=1
	// Linux/macOS:
	// export TEST_UPDATE_BSL=1

[vzarytovskii]

It used to not rely. That was the whole point of it, so when you run tests, you update it and it was part of PR and reflected there.

[vzarytovskii]

It was always updating it, I am not sure why would we make it not to:

4c60c63

[vzarytovskii]

This won't work if building with build scripts. The dotnet building flag is not officially supported with arcade builds, and will yield cryptic errors.

This should instead explain how to update surface area with build scripts and separately with dotnet test, same as it is done with builds.

[vzarytovskii]

Why is it here twice?

[smoothdeveloper]

First pass, the test will fail (and update the baseline), second pass, it is green.

[vzarytovskii]

But second isn't necessarily needed probably, should mention that and say that second one is optional.