Update some CMakeListsCustom.txt templates

Change-Id: I3d4d5d160bfa08f1837ca571c2ff29241893cb2e
Reviewed-on: https://code.wireshark.org/review/27685
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>

CMakeListsCustom.txt.example

@@ -12,7 +12,7 @@
 set(CUSTOM_PLUGIN_SRC_DIR
 #	private_plugins/foo
 # or
-#	plugins/foo
+#	plugins/epan/foo
 )
 
 #

epan/dissectors/CMakeListsCustom.txt.example

@@ -9,11 +9,11 @@
 # You can add custom builtin dissectors here by replaceing foo with your dissectors name.
 
 set(CUSTOM_DISSECTOR_SRC
-#	packet-foo.c
+#	${CMAKE_CURRENT_SOURCE_DIR}/packet-foo.c
 )
 
 set(CUSTOM_DIRTY_ASN1_DISSECTOR_SRC
-#	packet-bar.c
+#	${CMAKE_CURRENT_SOURCE_DIR}/packet-bar.c
 )
 
 #