t3200: clarify description of --set-upstream test

t3200: clarify description of --set-upstream test

Support for the --set-upstream option was removed in 52668846ea
(builtin/branch: stop supporting the "--set-upstream" option,
2017-08-17). The change did not completely remove the command
due to an issue noted in the commit's log message.

So, a test was added to ensure that a command which uses the
'--set-upstream' option fails instead of silently acting as an alias
for the '--set-upstream-to' option due to option parsing features.

To avoid confusion, clarify that the option is disabled intentionally
in the corresponding test description.

The test is expected to be around as long as we intentionally fail
on seeing the '--set-upstream' option which in turn we expect to
do for a period of time after which we can be sure that existing
users of '--set-upstream' are aware that the option is no
longer supported.

Signed-off-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>