doc: mention change in append behavior
doc: mention change in append behavior

Introduced in CL 347917.

Change-Id: I99b34341f787e779bd45b967110e70a035fa6558
Reviewed-on: https://go-review.googlesource.com/c/go/+/386217
Trust: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
Run-TryBot: Keith Randall <8c99c3a9284e493be632950b84cd789d08ed3e9d@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Martin Möhrmann <54669547a225ff20cba8b75a4adca540eef25858@golang.org>
Trust: Martin Möhrmann <54669547a225ff20cba8b75a4adca540eef25858@golang.org>
Run-TryBot: Martin Möhrmann <54669547a225ff20cba8b75a4adca540eef25858@golang.org>

nawawi/go,golang/go,golang/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,golang/go,CAFxX/go