content: clarify return values and naked returns

Fixes golang/go#12768

Change-Id: I18740d3fa002dcd8cef3261177a064f824a5a60a
Reviewed-on: https://go-review.googlesource.com/15080
Reviewed-by: Burcu Dogan <jbd@google.com>
Reviewed-by: Rob Pike <r@golang.org>
1 file changed