improve docstring of pathof

[jannefiluren]

Added references to dirname and basename to the docstring of pathof. It was not immediately clear to me how to use the results of pathof without knowing the existence of dirname. Could simplify this pr to just show See also dirname and basename.

[stevengj]

CI failures seem unrelated?

[mbauman]

Nope, they're very related:

+ gmake check-whitespace
base/loading.jl:258:Use [`dirname`](@ref) to get the directory part and [`basename`](@ref) 
Error: trailing whitespace found in source file(s)
This can often be fixed with:
    git rebase --whitespace=fix HEAD~1
or
    git rebase --whitespace=fix master
and then a forced push of the correct branch
gmake: *** [Makefile:92: check-whitespace] Error 1