No documentation of dropping behaviour for std::fs::File

[alercah]

The dropping behaviour for std::fs::File is inherited through the sys::fs::File object, meaning that its documentation does not come through to the File type. As a result, there is no documentation of the behaviour of File and, in particular, the decision to swallow errors (likely the correct one, but it should nonetheless be documented).

[alercah]

(there is a sentence "Files are automatically closed when they go out of scope.", but this doesn't discuss error handling, and it does not accurately reflect the fact that a drop and going out of scope are not the same thing.)