Documentation: Adjusted Doxygen config

Removed "included by" graphs from the internal docs as they
were too large/wide to be useful.

Added "referenced by" in internal docs to link to where the
symbol is used in the code.

doomsday/doomsday.doxy

@@ -116,7 +116,7 @@ FILTER_SOURCE_PATTERNS =
 SOURCE_BROWSER         = YES
 INLINE_SOURCES         = NO
 STRIP_CODE_COMMENTS    = YES
-REFERENCED_BY_RELATION = NO
+REFERENCED_BY_RELATION = YES
 REFERENCES_RELATION    = NO
 REFERENCES_LINK_SOURCE = YES
 USE_HTAGS              = NO
@@ -268,7 +268,7 @@ GROUP_GRAPHS           = YES
 UML_LOOK               = NO
 TEMPLATE_RELATIONS     = NO
 INCLUDE_GRAPH          = YES
-INCLUDED_BY_GRAPH      = YES
+INCLUDED_BY_GRAPH      = NO
 CALL_GRAPH             = NO
 CALLER_GRAPH           = NO
 GRAPHICAL_HIERARCHY    = YES

doomsday/engine/api.doxy

@@ -1,11 +1,13 @@
 # Public API documentation for libdeng
 @INCLUDE = ../doomsday.doxy
 
-PROJECT_NAME        = "libdeng"
-PROJECT_NUMBER      = 1.9.7
-PROJECT_BRIEF       = "Public API"
-OUTPUT_DIRECTORY    = ../apidoc/api/
+PROJECT_NAME            = "libdeng"
+PROJECT_NUMBER          = 1.9.7
+PROJECT_BRIEF           = "Public API"
+OUTPUT_DIRECTORY        = ../apidoc/api/
 
-INPUT               = api 
+INPUT                   = api 
+PREDEFINED              = __DOOMSDAY__
 
-PREDEFINED          = __DOOMSDAY__
+INCLUDED_BY_GRAPH       = YES
+REFERENCED_BY_RELATION  = NO