The Doxygen output was previously titled "OpenOCD Reference Manual",
which was quite misleading ... the User's Guide is the reference
manual which folk should consult about how to use the software.

Just rename it to match how it's been discussed previously, and to
bring out its intended audience:  developers of this software.  As a
rule, Doxygen is only for folk who work with the code it documents.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>