docs/manual: update pkg-kconfig doc about <pkg>-diff-config

Briefly states what the output of this target is about.

Signed-off-by: Marcel Patzlaff <m.patzlaff@pilz.de>
[Thomas: improve wording]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/manual/adding-packages-kconfig.txt

@@ -43,6 +43,12 @@ This snippet creates the following make targets:
   options that differ from the default values. It is not possible to
   use this target when fragment files are set.
 
+* +foo-diff-config+, which outputs the differences between the current
+  configuration and the one defined in the Buildroot configuration for
+  this kconfig package. The output is useful to identify the
+  configuration changes that may have to be propagated to
+  configuration fragments for example.
+
 and ensures that the source configuration file is copied to the build
 directory at the right moment.