[bdbarnett]

Part of #185.

put -r copies like cp -r: into a destination folder that already exists, the source folder goes inside it. That's unchanged here, but nothing said so, and re-uploading a library to the same path wrote a nested copy while the old files stayed where code imports them from.

• put --help, the CLI reference and the file-transfer guide now say where the folder goes, and the guide shows how to update a folder already on the board (put -r ./cast /).
• A destination whose parent folder doesn't exist failed with [Errno 2] /repro/cast/a.py, naming a file inside it. It now says can't create /repro/cast: the folder /repro isn't on the board (make it first: mpftp mkdir /repro), before writing anything.

Reproduced on an ESP32-P4 over USB: the first put into a missing /repro failed naming /repro/cast/a.py, and a second put -r new/cast /repro/cast wrote /repro/cast/cast/a.py. After the change the first names /repro, and put -r new/cast /repro replaced /repro/cast/a.py (read back as the new file). test_put.py has a test for the error that fails without the change.

A way to update a folder in place with the same destination path (a trailing slash, or a flag) would change what put -r does, so it isn't in this PR.