Document how to specify additional rts configure script flags

Since !6803 (closed), some of the global configure script flags have been moved to the rts configure script. However, it has not been documented anywhere how to pass those flags to the build process without needing to write a custom hadrian user flavour. For instance, previously passing --disable-large-address-space to GHC configure would work, but now I need to pass 'stage1.rts.cabal.configure.opts += --configure-option=--disable-large-address-space' to hadrian.

I spent a bit of time to find the hadrian flag above; it should be documented somewhere to avoid wasting another GHC developer's time.