Added comment to fancy_toys.jl should_build_platform

`should_build_platform` returns true, e.g. for "x86_64-linux-gnu-cxx11" when ARGS = ["x86_64-linux-gnu-cxx11-cuda+10.2"].

fancy_toys.jl

@@ -14,6 +14,9 @@ Return whether the tarballs for the given `platform` should be built.
 This is useful when the builder has different platform-dependent elements
 (sources, script, products, etc...) that make it hard to have a single
 `build_tarballs` call.
+
+Note that the platform must be augmented, e.g. wrt. CUDA, if the platform
+supplied in ARGS is augmented.
 """
 function should_build_platform(platform)
     # If you need inspiration for how to use this function, look at the builder