Clarify that VirtualBox guest utilities only work in 32-bit virtual machines

Originally created by @goupille on #11848 (Redmine)

two whisperback reports pointed that virtualbox-guest-utils does not work anymore since Tails 2.6. There was an upgrade of the package (now 5.0.24), but maybe it was not enough to work with linux-image-4.6* because le logs are saying :

amnesia virtualbox-guest-utils[1746]: Starting VirtualBox AdditionsNo suitable module for running kernel found ... failed!
amnesia virtualbox-guest-utils[1746]: failed!
amnesia systemd[1]: virtualbox-guest-utils.service: control process exited, code=exited status=1
amnesia systemd[1]: Failed to start LSB: VirtualBox Linux Additions.
-- Subject: Unit virtualbox-guest-utils.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
-- 
-- Unit virtualbox-guest-utils.service has failed.
-- 
-- The result is failed.

Feature Branch: emmapeel:docs/11848-clarify-VirtualBox-32bits

Related issues

Related to #11992 (closed)
Related to #11965 (closed)
Related to #12001 (closed)

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