Automatic link generation by doxygen

Nowadays, we have one page on "GDB Types" generated by doxygen, but types
and macros referenced in doc are not linked to their definitions.  This
patch tweaks the comments a little to use doxygen syntax so that these
types and macros are linked their definitions.

Is it OK?

gdb:

2014-04-17  Yao Qi  <yao@codesourcery.com>

	* gdbtypes.h: Update comments to link to types and macros'
	definitions.