gdb/doc: don't rely on @menu item within the docs

gdb/doc: don't rely on @menu item within the docs

The node 'Auto-loading extensions' currently relies on a @menu item to
provide a set of cross-references to different parts of the manual.
Additionally the menu is placed part way through the node and the text
prior to the menu seems (to me) to assume that the menu will be
formatted into the document.

This is a bad idea as the menus are not always part of the final
document (e.g. pdf output does not include the menu), when compared to
the info page the pdf version of this node is less helpful as it lacks
proper cross references.  Menus should always be placed at the end of
a node.

In this commit I rewrite a paragraph to add extra cross references
inline within the text.  I then move the menu to the end of the node.

gdb/doc/ChangeLog:

	* gdb.texinfo (Auto-loading extensions): Add additional cross
	references and move @menu to the end of the node.