Clocks userguide: add a bit more structure

Modelica/Clocked/UsersGuide/Clocks.mo

@@ -32,11 +32,20 @@ Notice that a clocked variable has <em>only a value when the corresponding clock
 <img src=\"modelica://Modelica/Resources/Images/Clocked/Clocks/clockSignals.png\" alt=\"Clock variables and clocked variables\">
 </div>
 
+<h3>Clock blocks and connectors</h3>
+<p>
+The Clock type introduced above is a base type of the Modelica language
+and it is mostly intended for textual model description.
+The Clocked library encapsulates this base type and allows for
+graphical modeling thanks to the clock blocks and connectors it provides.
+</p>
 <p>
 Similarly to RealInput, RealOutput etc., clock input and output connectors, called ClockInput and ClockOutput, are defined in sublibrary
 <a href=\"modelica://Modelica.Clocked.ClockSignals.Interfaces\">ClockSignal.Interfaces</a>
-in order to propagate clocks via connections. A clock signal can be generated with
-one of the blocks of sublibrary
+in order to propagate clocks via connections.
+</p>
+<p>
+A clock signal can be generated with one of the blocks of sublibrary
 <a href=\"modelica://Modelica.Clocked.ClockSignals.Clocks\">ClockSignals.Clocks</a>:
 </p>
 
@@ -49,6 +58,7 @@ The output signals of the blocks in the above figure are clock signals,
 by default visualized with dotted grey lines.
 </p>
 
+<h3>Derived clocks (sub/super/shift-sampling)</h3>
 <p>
 With the blocks of sublibrary
 <a href=\"modelica://Modelica.Clocked.ClockSignals.Sampler\">ClockSignals.Sampler</a>
@@ -99,7 +109,7 @@ This means it is <em>not allowed to mix continuous time and discrete-time operat
 (unlike in the global continuous time partition).
 </p>
 </p>
-Also notice that in a clocked discrete-time partition all event generating mechanisms
+Also notice that in a clocked discrete-time partition all event-generating mechanisms
 do no longer apply. Especially neither relations, nor one of the built-in event
 triggering operators will trigger an event.
 </p>