doc: Clarify what <string> means in path option of fs.readFile, fs.readFileSync etc.

- **Version**: 14.15.3 x64 - **Platform**: Windows 10 64-bit - **Subsystem**: fs Affected URL(s): https://nodejs.org/dist/latest-v14.x/docs/api/fs.html#fs_fs_readfile_path_options https://nodejs.org/dist/latest-v14.x/docs/api/fs.html#fs_fs_readfilesync_path_options ## Description I am not sure if this a documentation issue or a bug. I get a ENOENT error when I use a url string as path for fs.readFile. However, if I convert the url string to a file path, I am able to read the file. It needs to be clarified if path strings are exclusively file paths and not urls. If it can also be a url string (and I see no reason for not supporting them?) , then this is bug/feature request!