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.

man/systemd.unit.xml

@@ -164,7 +164,7 @@
     instance (if any) is always uniquely defined for a given unit and all its aliases. The target of alias
     symlink must point to a valid unit file location, i.e. the symlink target name must match the symlink
     source name as described, and the destination path must be in one of the unit search paths, see UNIT FILE
-    LOAD PATH section below for more details. Note that the target file may not exist, i.e. the symlink may
+    LOAD PATH section below for more details. Note that the target file might not exist, i.e. the symlink may
     be dangling.</para>
 
     <para>Unit files may specify aliases through the <varname>Alias=</varname> directive in the [Install]
@@ -196,7 +196,7 @@
     <filename>.wants/</filename> or <filename>.requires/</filename> must thus point to a valid unit file
     location, i.e. the symlink target name must satisfy the described requirements, and the destination path
     must be in one of the unit search paths, see UNIT FILE LOAD PATH section below for more details. Note
-    that the target file may not exist, i.e. the symlink may be dangling.</para>
+    that the target file might not exist, i.e. the symlink may be dangling.</para>
 
     <para>Along with a unit file <filename>foo.service</filename>, a "drop-in" directory
     <filename>foo.service.d/</filename> may exist. All files with the suffix