version ghcpkgdocdir and add new html doc dir names
    
    (ghcpkgdocdir used by JuicyPixels)
    
    in future:
    
    %ghcdocbasedir -> %ghc_html_dir
    %ghclibdocdir -> %ghc_html_libraries_dir
    %ghcpkgdocdir -> %ghc_html_libraries_dir
    
    Hopefully the new names should be clearer