Document that the built-in open() when called with a file descriptor ignores "mode" argument

At https://docs.python.org/3/library/functions.html#open, in the documentation of the mode argument there is no information that this argument is ignored when the function is called with a file descriptor instead of a path.

See Why does os.fdopen ignore mode? SO question. The question talks about os.fdopen() but according to https://docs.python.org/3/library/os.html#os.fdopen

This is an alias of the open() built-in function and accepts the same arguments.

so I think it should be enough to place the information in the documentation of the mode argument of the built-in open() function. This way it would document the built-in open() function but also the os.fdopen() function by virtue of the latter being an alias of the former.

Start at the linked built-in open() documentation and inspect the mode argument description. Check the linked os.fdopen() entry to confirm the alias context, then document that mode is ignored when open() receives a file descriptor; the open() and os.fdopen() documentation should clearly convey this behavior.