Installing additional documentation files

[Ryan Schmidt]

I added a portfile recipe for installing additional documentation files:

https://trac.macports.org/wiki/PortfileRecipes#doc

I recommended that documentation should go in ${prefix}/share/doc/$ 
{name}. This is where we always used to install additional  
documentation files.

A year or two ago, Anthony started changing things to install those  
files in ${prefix}/share/doc/${name}-${version} instead, but I didn't  
really ever understand the reasoning. You can't ever have more than  
one version of a port active at the same time, so it's not like $ 
{prefix}/share/doc/${name} can cause a conflict. Does anybody have an  
objection to returning to the old recommendation?

If not, then I would also like to have MacPorts base auto-create $ 
{prefix}/share/doc/${name} before the destroot phase, so each port  
doesn't have to "xinstall -d" it manually. MacPorts base already  
creates ${prefix}/share/${name} automatically so I think this fits.