man/systemd.timer: Correct inaccuracy in man page

The docs previously stated that RandomizedDelaySec is applied onto the
next scheduled time, but after 9fa326b
this is no longer the case.

I also reworded FixedRandomDelay= slightly, to make it a bit clearer

(cherry picked from commit fca20a1be69d0ffe3067e5c8676d75b5f8af1a5d)
(cherry picked from commit 0930949e64cd91e366865e104a295ddc83170fdd)
(cherry picked from commit a37d96386a3f8ded382164ad85e458e848853499)
(cherry picked from commit fe859ba)

Author: AdrianVovk

man/systemd.timer.xml

@@ -260,9 +260,9 @@
 
         <listitem><para>Delay the timer by a randomly selected, evenly distributed amount of time between 0
         and the specified time value. Defaults to 0, indicating that no randomized delay shall be applied.
-        Each timer unit will determine this delay randomly before each iteration, and the delay will simply
-        be added on top of the next determined elapsing time, unless modified with
-        <varname>FixedRandomDelay=</varname>, see below.</para>
+        Each timer unit will determine this delay randomly before each iteration, unless modified with
+        <varname>FixedRandomDelay=</varname>, see below. The delay is added on top of the next determined
+        elapsing time or the service manager's startup time, whichever is later.</para>
 
         <para>This setting is useful to stretch dispatching of similarly configured timer events over a
         certain time interval, to prevent them from firing all at the same time, possibly resulting in
@@ -284,12 +284,12 @@
       <varlistentry>
         <term><varname>FixedRandomDelay=</varname></term>
 
-        <listitem><para>Takes a boolean argument. When enabled, the randomized offset specified by
-        <varname>RandomizedDelaySec=</varname> is reused for all firings of the same timer. For a given timer
-        unit, the offset depends on the machine ID, user identifier and timer name, which means that it is
-        stable between restarts of the manager. This effectively creates a fixed offset for an individual
-        timer, reducing the jitter in firings of this timer, while still avoiding firing at the same time as
-        other similarly configured timers.</para>
+        <listitem><para>Takes a boolean argument. When enabled, the randomized delay specified by
+        <varname>RandomizedDelaySec=</varname> is chosen deterministically, and remains stable between all
+        firings of the same timer, even if the manager is restarted. The delay is derived from the machine
+        ID, the manager's user identifier, and the timer unit's name. This effectively creates a unique fixed
+        offset for each timer, reducing the jitter in firings of an individual timer while still avoiding
+        firing at the same time as other similarly configured timers.</para>
 
         <para>This setting has no effect if <varname>RandomizedDelaySec=</varname> is set to 0. Defaults to
         <option>false</option>.</para></listitem>