NMake: Document response file encoding heuristic in a comment

Since commit f3f57cc4ed (NMake: Use UTF-8 with BOM if supported by
nmake, 2021-04-22, v3.21.0-rc1~217^2) the encoding of response files is
selected based on the makefile encoding.  In principle these may be
orthogonal, but in practice it is a useful heuristic.  Call out this
heuristic in a comment, and leave a FIXME to do something better.