This fixes some ambiguous wording in a comment in
`_safer_popen_wording`, where it was unclear if the secondary
problem -- where it would be possible to run a wrong `cmd.exe`-type
shell -- would happen under two separate circumstances, or only
when both circumstances occurred together. This adjusts its wording
to make clear that it is the latter.

This also fixes a minor typo in another `_safer_popen_windows`
comment.

This might be viewed as building on the improvements in b9d9e56
(gitpython-developers#1859), but the changes here are to comments only.