Add documention on how to Mount a Host File as a Data Volume

In #6011 it was proposed to update the documentation to warn about inode changes for **single-file** bind mounts.
I think this sentence can be simplified in the following way:

 - The `-v` flag can also be used to mount a single file - instead of *just* directories - from the host machine.

However, please feel free to ignore if other MAINTAINERS think it's fine.