nginx: update Passenger caveats.

They now explicitly mention the fact that passenger_root and passenger_ruby
must be specified in the 'http' context. See
http://stackoverflow.com/questions/24220300/configuring-passenger-with-ngenix
for a user who didn't get this and did it wrong.

Closes #30155.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>