Re-title Developer's Guide

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>

doc/manual/main.txt

@@ -1,6 +1,6 @@
-/** @mainpage OpenOCD Reference Manual
+/** @mainpage OpenOCD Developer's Guide
 
-Welcome to the OpenOCD Reference Manual -- the developer's resource for
+Welcome to the OpenOCD Developer's Guide -- the developer's resource for
 learning about the internal architecture of the OpenOCD project. @par
 
 In addition, this document contains the tactical and strategic plans