Re #6679 Document current setup-depends behaviour, re missing Cabal

Author: mpilgrem

doc/commands/build_command.md

@@ -210,6 +210,12 @@ part of Stack's
 [user's guide](../tutorial/building_your_project.md#adding-dependencies) for
 information about how these dependencies get specified.
 
+If a package description specifies a custom build type, it must also specify a
+custom setup. That should list the dependencies needed to compile `Setup.hs`.
+Stack further customises the setup, using the `Cabal` package. If that package
+is not listed, Stack will not add the package as a dependency but warn that
+build errors are likely.
+
 In addition to specifying targets, you can also control what gets built, or
 retained, with the flags and options listed below. You can also affect what gets
 built by specifying Cabal (the library) options for the configure step