doc/reftable: document how to handle windows

doc/reftable: document how to handle windows

On Windows we can't delete or overwrite files opened by other processes. Here we
sketch how to handle this situation.

We propose to use a random element in the filename. It's possible to design an
alternate solution based on counters, but that would assign semantics to the
filenames that complicates implementation.

Signed-off-by: Han-Wen Nienhuys <redacted>
Signed-off-by: Junio C Hamano <redacted>