CodingGuidelines: document a shell that "fails" "VAR=VAL shell_func"

Over the years, we accumulated the community wisdom to avoid the
common "one-short export" construct for shell functions, but seem to
have lost on which exact platform it is known to fail.  Now during
an investigation on a breakage for a recent topic, we found one
example of failing shell.  Let's document that.

This does *not* mean that we can freely start using the construct
once Ubuntu 20.04 is retired.  But it does mean that we cannot use
the construct until Ubuntu 20.04 is fully retired from the machines
that matter.  Moreover, posix explicitly says that the behaviour for
the construct is unspecified.

Helped-by: Kyle Lippincott <spectral@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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