man: document ShowStatus and SetShowStatus()

SetShowStatus() was added in order to fix #11447. Recently, I ran into
the exact same problem that OP was experiencing in #11447. I wasn’t able
to figure out how to deal with the problem until I found #11447, and it
took me a while to find #11447.

This commit takes what I learned from reading #11447 and adds it to the
documentation. Hopefully, this will make it easier for other people who
run into the same problem in the future.

man/systemd.exec.xml

@@ -3001,7 +3001,12 @@ SystemCallErrorNumber=EPERM</programlisting>
 
         <para><option>tty</option> connects standard output to a tty (as configured via <varname>TTYPath=</varname>,
         see below). If the TTY is used for output only, the executed process will not become the controlling process of
-        the terminal, and will not fail or wait for other processes to release the terminal.</para>
+        the terminal, and will not fail or wait for other processes to release the terminal. Note: if a unit
+        tries to print multiple lines to a TTY during bootup or shutdown, then there's a chance that those
+        lines will be broken up by status messages. <function>SetShowStatus()</function> can be used to
+        prevent this problem. See
+        <citerefentry project="man-pages"><refentrytitle>org.freedesktop.systemd1</refentrytitle><manvolnum>5</manvolnum></citerefentry>
+        for details.</para>
 
         <para><option>journal</option> connects standard output with the journal, which is accessible via
         <citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>. Note