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/gtype.c

@@ -48,7 +48,7 @@
  *     management system
  * @title:Type Information
  *
- * The GType API is the foundation of the GObject system.  It provides the
+ * The GType API is the foundation of the GObject system. It provides the
  * facilities for registering and managing all fundamental data types,
  * user-defined object and interface types.
  *