crypto/tls,crypto/x509: clarify certificate ownership

crypto/tls,crypto/x509: clarify certificate ownership

Clarify documentation in cases where certificates returned from
various methods are not owned by the caller, and as such should not
be modified.

Change-Id: I06bdc4cf0f686c3d5e8bbb76fc71f2a4bdb955e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/427155
Auto-Submit: Roland Shoemaker <roland@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>