update copy documentation to show differences with cp

[nicolas-marien]

A found myself struggling because I was using the copy function with file source and directory destination (as you can do using cp). The error was related to an unlink system call not working.
After some digging I landed on #323 but I found that the docs has not been updated.

So here are a few words about this topic.
Thank you :)

[coveralls]

Coverage remained the same at 87.377% when pulling 4f01664 on nico29:docs/copy-improvements into 1dd5c18 on jprichardson:master.

[jprichardson]

Thank you @nico29!