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