Document the value of long-running variants for bug-finding

[afd]

A variant shader, being large, may take many seconds to render an image. This might give the impression that bugs induced by a variant shader are irrelevant, since realistic shaders are not allowed to take many seconds to render.

However, in practice, a bug exposed by a large variant shader can typically be reproduced by a small, fast-running shader after running the reducer. It is thus worth finding and reducing bugs found by large variant shaders, as they typically can be triggered by more realistic, small, fast-running shaders.

We should make this clearer in the tool's documentation.

[shivanshi12402]

I wish to work on this project. Can someone please guide me further?