document toolchainopts

[boegel]

Document toolchainopts (once --avail-toolchainopts is supported), mention that they don't make sense in combination with a dummy toolchain

[ocaisa]

+1 for this

[pescobar]

I have arrived here looking for the documentation for toolchainopts ;)

+2 for this

[boegel]

fixed in easybuilders/easybuild-framework#1830, see e.g. eb --avail-toolchain-opts intel and http://easybuild.readthedocs.io/en/latest/version-specific/toolchain_opts.html