Add docs about creating parent folders (#330)

Clarify in documentation that write_files will create parent folders
for paths that do not already exist.

This obfuscates what the problem is when people erroneously create
files in /tmp despite the warnings in the documentation not to do so.
People naturally assume that their file is absent because the parent folder
did not exist for it to be created in, causing them to add a runcmd block
to create the folder, even though execution order means that this will not
occur until after write_files have all finished.

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