doc: reference --keep-file-descriptors for "bundle exec"

"bundle exec" alone is not suitable for use with systemd-style
socket activation due to Ruby 2.0+ behavior of setting close-on-exec
for file descriptors above 2.  However, the "--keep-file-descriptors"
option was added to bundler 1.4.0 to workaround this Ruby 2.0 change
and may be used to prevent Ruby 2.0+ from closing file descriptors
on exec.

Thanks to Amir Yalon and Christos Trochalakis for bringing up
this issue on the mailing list:

http://bogomips.org/unicorn-public/1457824748.3666627.547425122.2A828B07@webmail.messagingengine.com/

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