[IMP] qweb doc: debugging directives

fixes #4547

doc/reference/qweb.rst

@@ -335,6 +335,17 @@ website's rich text edition.
 ``t-field-options`` can be used to customize fields, the most common option
 is ``widget``, other options are field- or widget-dependent.
 
+debugging
+---------
+
+``t-debug``
+    invokes a debugger using PDB's ``set_trace`` API. The parameter should
+    be the name of a module, on which a ``set_trace`` method is called::
+    
+        <t t-debug="pdb"/>
+    
+    is equivalent to ``importlib.import_module("pdb").set_trace()``
+
 Helpers
 -------
 
@@ -477,13 +488,30 @@ The javascript QWeb implementation provides a few debugging hooks:
 
 ``t-log``
     takes an expression parameter, evaluates the expression during rendering
-    and logs its result with ``console.log``
+    and logs its result with ``console.log``::
+    
+        <t t-set="foo" t-value="42"/>
+        <t t-log="foo"/>
+        
+    will print ``42`` to the console
 ``t-debug``
-    triggers a debugger breakpoint during template rendering
+    triggers a debugger breakpoint during template rendering::
+    
+        <t t-if="a_test">
+            <t t-debug="">
+        </t>
+
+    will stop execution if debugging is active (exact condition depend on the
+    browser and its development tools)
 ``t-js``
     the node's body is javascript code executed during template rendering.
     Takes a ``context`` parameter, which is the name under which the rendering
-    context will be available in the ``t-js``'s body
+    context will be available in the ``t-js``'s body::
+    
+        <t t-set="foo" t-value="42"/>
+        <t t-js="ctx">
+            console.log("Foo is", ctx.foo);
+        </t>
 
 Helpers
 -------