Clean up the documetation.

Docs/GettingStarted.dox

@@ -125,7 +125,7 @@ Note that you can also force an OpenGL mode build on Windows by using the CMake
 
 If ENABLE_DOCS build option is set then a normal (ALL) build would not only build Urho3D software but also Urho3D documentation automatically. If it is not then the documentation can be generated by manually invoking 'make doc' command or its equivalent command in IDE.
 
-The prerequisite is Doxygen and (optionally) standalone Lua interpreter to dump the \ref LuaScriptAPI "LuaScript API" when the LuaScript subsystem is also enabled. A tool to dump the \ref ScriptAPI "AngelScript API" for the default \ref Script "scripting" subsystem will be built internally when all the tools are being built.  
+The prerequisite are Doxygen, Graphviz, and (optionally) standalone Lua interpreter to dump the \ref LuaScriptAPI "LuaScript API" when the LuaScript subsystem is also enabled. A tool to dump the \ref ScriptAPI "AngelScript API" for the default \ref Script "scripting" subsystem will be built internally when all the tools are being built.  
 
 \section Build_Options Build options
 

Readme.txt

@@ -101,8 +101,10 @@ Documentation
 
 Urho3D classes have been sparsely documented using Doxygen notation. To
 generate documentation into the "Docs" subdirectory, open the Doxyfile in the
-root directory with doxywizard and click "Run doxygen" from the "Run" tab.
-Get Doxygen from http://www.doxygen.org
+"Docs" subdirectory with doxywizard and click "Run doxygen" from the "Run" tab.
+Get Doxygen from http://www.doxygen.org & Graphviz from http://www.graphviz.org.
+See section "Documentation build" below on how to automate documentation
+generation as part of the build process.
 
 The documentation is also available online at http://cadaver.homeftp.net/urho3d
 
@@ -377,10 +379,10 @@ build Urho3D software but also Urho3D documentation automatically. If it is not
 then the documentation can be generated by manually invoking 'make doc' command
 or its equivalent command in IDE.
 
-The prerequisite is Doxygen and (optionally) standalone Lua interpreter to dump
-the LuaScript API when the LuaScript subsystem is also enabled. A tool to dump
-the AngelScript API for the default scripting subsystem will be built internally
-when all the tools are being built.
+The prerequisite are Doxygen, Graphviz, and (optionally) standalone Lua
+interpreter to dump the LuaScript API when the LuaScript subsystem is also
+enabled. A tool to dump the AngelScript API for the default scripting subsystem
+will be built internally when all the tools are being built.
 
 
 Build options