os: document File's concurrent operation resource limits

Document that *os.File is subject to resource limits
for concurrent operations. We aren't documenting
a specific number of concurrent operations because that
number is OS/system dependent. This limit comes from:
    internal/poll/fd_mutex.go
where we use 20 bits to count locks.

Fixes #32544

Change-Id: I7d305d4aaba5b2dbc6f1ab8c447117fde5e31a66
Reviewed-on: https://go-review.googlesource.com/c/go/+/181841


Reviewed-by: Rob Pike <r@golang.org>