os: document CopyFS behavior for symlinks in destination

Also clarify the permissions of created files,
and note that CopyFS will not overwrite files.

Update a few places in documentation to use 0oXXX for octal consts.

For #62484

Change-Id: I208ed2bde250304bc7fac2b93963ba57037e791e
Reviewed-on: https://go-review.googlesource.com/c/go/+/600775


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>