docs: deprecated.rst: Expand str*cpy() replacement notes

The notes on replacing the deprecated str*cpy() functions didn't call
enough attention to the change in return type. Add these details and
clean up the language a bit more.

Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20201015231730.2138505-1-keescook@chromium.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>