Building the LO Flatpak for --arch=i386 (where CFLAGS and CXXFLAGS are both set
to "-march=i686 -mtune=generic -O2 -g -pipe -Wp,-D_FORTIFY_SOURCE=2
-Wp,-D_GLIBCXX_ASSERTIONS -fexceptions -fstack-protector-strong
-grecord-gcc-switches -fasynchronous-unwind-tables -fstack-clash-protection
-fcf-protection").  Even if that was due to a glitch of not disabling tests
wholesale for that arch (which has meanwhile been addressed with
<https://github.com/flathub/org.libreoffice.LibreOffice/pull/67/commits/
deda15380b436ca8fe62e2ebc1feec8bcb6dd43e> "org.freedesktop.Sdk//18.08 `uname -i`
always returns 'unknown'"), lets document that failing corner case here.

Change-Id: I8f78cced45e015331e23ae9c86bc286179278076
Reviewed-on: https://gerrit.libreoffice.org/66904
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>