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/gtypeplugin.c

@@ -26,9 +26,10 @@
  * @see_also: #GTypeModule and g_type_register_dynamic().
  * @title: GTypePlugin
  *
+ * An interface that handles the lifecycle of dynamically loaded types.
+ *
  * The GObject type system supports dynamic loading of types.
- * The #GTypePlugin interface is used to handle the lifecycle
- * of dynamically loaded types. It goes as follows:
+ * It goes as follows:
  *
  * 1. The type is initially introduced (usually upon loading the module
  *    the first time, or by your main application that knows what modules