docs: Start stanzas with a single paragraph

When rendering the contents of the GLib documentation stored inside the
introspection data, a common behaviour is to take the first paragraph as
a summary of the symbol being documented.

The documentation is assumed to be in Markdown format, which means:

 - paragraphs must be separated by newlines
 - lines that have an indentation of four or more spaces are considered
   code blocks
 - lines that start with a `#` are considered titles

This means we need to slightly tweak the documentation in our sources to
ensure that it can be rendered appropriately by tools that are not
gtk-doc.

See issue: #2365

gobject/gobject.c

@@ -66,15 +66,18 @@
  * claimed to be "owned" by any code portion. The main motivation for
  * providing floating references is C convenience. In particular, it
  * allows code to be written as:
+ * 
  * |[<!-- language="C" --> 
  * container = create_container ();
  * container_add_child (container, create_child());
  * ]|
+ * 
  * If container_add_child() calls g_object_ref_sink() on the passed-in child,
  * no reference of the newly created child is leaked. Without floating
  * references, container_add_child() can only g_object_ref() the new child,
  * so to implement this code without reference leaks, it would have to be
  * written as:
+ *
  * |[<!-- language="C" --> 
  * Child *child;
  * container = create_container ();
@@ -82,6 +85,7 @@
  * container_add_child (container, child);
  * g_object_unref (child);
  * ]|
+ *
  * The floating reference can be converted into an ordinary reference by
  * calling g_object_ref_sink(). For already sunken objects (objects that
  * don't have a floating reference anymore), g_object_ref_sink() is equivalent
@@ -537,11 +541,13 @@ g_object_do_class_init (GObjectClass *class)
    * This signal is typically used to obtain change notification for a
    * single property, by specifying the property name as a detail in the
    * g_signal_connect() call, like this:
+   *
    * |[<!-- language="C" --> 
    * g_signal_connect (text_view->buffer, "notify::paste-target-list",
    *                   G_CALLBACK (gtk_text_view_target_list_notify),
    *                   text_view)
    * ]|
+   *
    * It is important to note that you must use
    * [canonical parameter names][canonical-parameter-names] as
    * detail strings for the notify signal.
@@ -4530,11 +4536,12 @@ g_initially_unowned_class_init (GInitiallyUnownedClass *klass)
 /**
  * GWeakRef:
  *
- * A structure containing a weak reference to a #GObject.  It can either
- * be empty (i.e. point to %NULL), or point to an object for as long as
- * at least one "strong" reference to that object exists. Before the
- * object's #GObjectClass.dispose method is called, every #GWeakRef
- * associated with becomes empty (i.e. points to %NULL).
+ * A structure containing a weak reference to a #GObject.
+ *
+ * A `GWeakRef` can either be empty (i.e. point to %NULL), or point to an
+ * object for as long as at least one "strong" reference to that object
+ * exists. Before the object's #GObjectClass.dispose method is called,
+ * every #GWeakRef associated with becomes empty (i.e. points to %NULL).
  *
  * Like #GValue, #GWeakRef can be statically allocated, stack- or
  * heap-allocated, or embedded in larger structures.