os: mention ErrInvalid for nil receivers

[dominikh]

Methods on *os.File return ErrInvalid when the receiver is nil. This is, however, not documented anywhere. Usually this would be fine, if it weren't for a lot of documentation on methods saying If there is an error, it will be of type *PathError.

Operating on nil *os.Files is a programming mistake and shouldn't be a common occurence, so it shouldn't complicate all of the documentation. However, a note in the package-level documentation could be an easy fix for a small inaccuracy in the rest of the documentation.

[gopherbot]

CL https://golang.org/cl/16873 mentions this issue.