Subsections in custom-setup

By reading http://cabal.readthedocs.io/en/latest/developing-packages.html#custom-setup-scripts it's not clear whether subsections e.g. `if os(windows)` are supported. IIRC they aren't, but IMHO it's good idea to explicitly mention this (and the reasoning behind).

Also: if such section is added, cabal crashes:

```hs
'parseField' called on a non-field. This is possibly a bug in Cabal.
Please report it to the developers:
https://github.com/haskell/cabal/issues/new
CallStack (from HasCallStack):
error, called at ./Distribution/PackageDescription/Utils.hs:21:16 in
```

(I'm not sure what parsec variant does).

Start with the custom-setup section in developing-packages.html and reproduce the shown if os(windows) example. Read Distribution/PackageDescription/Utils.hs around line 21 to understand the parseField failure; done means the supported subsection behavior and its reasoning are explicit and the reported input no longer causes the Cabal crash.

Written by the indexing model from the issue text.