man: use singular “they”

For an example where we already use it, see man:sd-login(3):

> A session is defined by the time a user is logged in until they log out.

As far as I can tell, this removes the only remaining occurrences of
referring to users by gendered pronouns in our documentation (though
some still survive in code comments and the NEWS and TODO files):

    git grep '\b\(he\|him\|his\|she\|her\|hers\)\b' man/

man/systemd-journald.service.xml

@@ -206,10 +206,10 @@ systemd-tmpfiles --create --prefix /var/log/journal</programlisting>
 
     <para>Journal files are, by default, owned and readable by the
     <literal>systemd-journal</literal> system group but are not
-    writable. Adding a user to this group thus enables her/him to read
+    writable. Adding a user to this group thus enables them to read
     the journal files.</para>
 
-    <para>By default, each logged in user will get her/his own set of
+    <para>By default, each logged in user will get their own set of
     journal files in <filename>/var/log/journal/</filename>. These
     files will not be owned by the user, however, in order to avoid
     that the user can write to them directly. Instead, file system

man/systemd.generator.xml

@@ -260,7 +260,7 @@
       <para><citerefentry><refentrytitle>systemd-fstab-generator</refentrytitle><manvolnum>8</manvolnum></citerefentry>
       converts <filename>/etc/fstab</filename> into native mount units. It uses
       argv[1] as location to place the generated unit files in order to allow the
-      user to override <filename>/etc/fstab</filename> with her own native unit
+      user to override <filename>/etc/fstab</filename> with their own native unit
       files, but also to ensure that <filename>/etc/fstab</filename> overrides any
       vendor default from <filename>/usr</filename>.</para>