disableFastEmit=false can cause unexpected compilation of typescript code

[akosbordas]

It is not obvious enough what kind of circumstances does it have to use fast emit feature. I ran into a problem which is described here #11202.

The guys from typescript suggest that:

transpileModule should not be used unless an error-free compilation of the same sources has occurred under the --isolatedModules flag, which verifies that it is safe to use transpileModule on a particular compilation.

It would be good if you could at least extend your documentation to describe what problems are caused by using fast mode.