Updates help info to indicate that options such as -g must be specified _after_ the command (as of version 5)

Several ghc-mod relevant vim extensions are broken due to this change in version 5, they have relied on being able to do:

    ghc-mod -g {ghcOpt} {command} …

which now gives

    ghc-mod: unknown command: `-g'

the command must be `ghc-mod {command} -g {ghcOpt}`