doc/go1.15: mention consequence of os.File.ReadFrom

Now that we've added a os.File.ReadFrom method, io.CopyBuffer to a
os.File will no longer use the provided buffer.

For #16474
For #36817
For #37419

Change-Id: I79a3bf778ff93eab88e88dd9ecbb8c7ea101e868
Reviewed-on: https://go-review.googlesource.com/c/go/+/238864


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>