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.

docs/ENVIRONMENT.md

@@ -341,7 +341,7 @@ All tools:
   for cases where we don't need to track given unit type, e.g. `--user` manager
   often doesn't need to deal with device or swap units because they are
   handled by the `--system` manager (PID 1). Note that setting certain unit
-  type as unsupported may not prevent loading some units of that type if they
+  type as unsupported might not prevent loading some units of that type if they
   are referenced by other units of another supported type.
 
 * `$SYSTEMD_DEFAULT_MOUNT_RATE_LIMIT_BURST` — can be set to override the mount