man: explain what kind of data is actually included in "systemd-analyze plot" output

Prompted by: https://lists.freedesktop.org/archives/systemd-devel/2023-January/048736.html

diff --git a/man/systemd-analyze.xml b/man/systemd-analyze.xml
index 0d91cdd7d06d4b73e220647fde07bddfab0b718c..4a276bd3c77ca4757fe93689a9173b4962137da0 100644 (file)
--- a/man/systemd-analyze.xml
+++ b/man/systemd-analyze.xml
@@ -284,6 +284,13 @@ Timestamp units-load-finish: Thu 2019-03-14 23:28:07 CET
 $ eog bootup.svg&
 </programlisting>
       </example>
+
+      <para>Note that this plot is based on the most recent per-unit timing data of loaded units. This means
+      that if a unit gets started, then stopped and then started again the information shown will cover the
+      most recent start cycle, not the first one. Thus it's recommended to consult this information only
+      shortly after boot, so that this distinction doesn't matter. Moreover, units that are not referenced by
+      any other unit through a dependency might be unloaded by the service manager once they terminate (and
+      did not fail). Such units will not show up in the plot.</para>
     </refsect2>
 
     <refsect2>