Document the substitutions in ldc2.conf

[JohanEngelen]

It'd be good to document the few substitution variables we have in the default configfile (ldc2.conf.in etc.).
%%ldcbinarypath%%
%%ldcconfigpath%%
%%ldcversion%% (#4812 )

Doesn't have to be a lot of text, just brief descriptions (and example string) would already be great to have.
Similar to the documentation of the cross-compilation target section matching.