kbuild: explain why DEBUG_SECTION_MISMATCH is UNDEFINED

We started to see patches enabling this - so explain why
it is disabled and the condition to enable it again.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

lib/Kconfig.debug

@@ -82,6 +82,9 @@ config HEADERS_CHECK
 config DEBUG_SECTION_MISMATCH
 	bool "Enable full Section mismatch analysis"
 	depends on UNDEFINED
+	# This option is on purpose disabled for now.
+	# It will be enabled when we are down to a resonable number
+	# of section mismatch warnings (< 10 for an allyesconfig build)
 	help
 	  The section mismatch analysis checks if there are illegal
 	  references from one section to another section.