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/gvalue.c

@@ -42,6 +42,7 @@
  *
  * The #GValue structure is basically a variable container that consists
  * of a type identifier and a specific value of that type.
+ *
  * The type identifier within a #GValue structure always determines the
  * type of the associated value.
  *