docs,man: Avoid some ambiguous uses of "may not"

docs,man: Avoid some ambiguous uses of "may not"

Like much English text, the systemd documentation uses "may not" in the
sense of both "will possibly not" and "is forbidden to".  In many cases
this is OK because the context makes it clear, but in others I felt it
was possible to read the "is forbidden to" sense by mistake: in
particular, I tripped over "the target file may not exist" in
systemd.unit(5) before realizing the correct interpretation.

Use "might not" or "may choose not to" in these cases to make it clear
which sense we mean.