Document when to use Windows' `symlink_dir` vs. `symlink_file`
Document when to use Windows' `symlink_dir` vs. `symlink_file`

It was previously unclear which should be used when.

aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust