package/libsigrok: explain why host-doxygen is needed

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libsigrok/libsigrok.mk

@@ -53,6 +53,7 @@ endif
 
 ifeq ($(BR2_PACKAGE_LIBSIGROKCXX),y)
 LIBSIGROK_CONF_OPTS += --enable-cxx
+# host-doxygen is used by C++ bindings to parse libsigrok symbols
 LIBSIGROK_DEPENDENCIES += \
 	glibmm \
 	host-doxygen \