kbuild: doc: document subdir-y syntax

There is no explanation about subdir-y.

Let's document it.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>

Documentation/kbuild/makefiles.rst

@@ -319,6 +319,21 @@ more details, with real examples.
 	that directory specifies obj-y, those objects will be left orphan.
 	It is very likely a bug of the Makefile or of dependencies in Kconfig.
 
+	Kbuild also supports dedicated syntax, subdir-y and subdir-m, for
+	descending into subdirectories. It is a good fit when you know they
+	do not contain kernel-space objects at all. A typical usage is to let
+	Kbuild descend into subdirectories to build tools.
+
+	Examples::
+
+		# scripts/Makefile
+		subdir-$(CONFIG_GCC_PLUGINS) += gcc-plugins
+		subdir-$(CONFIG_MODVERSIONS) += genksyms
+		subdir-$(CONFIG_SECURITY_SELINUX) += selinux
+
+	Unlike obj-y/m, subdir-y/m does not need the trailing slash since this
+	syntax is always used for directories.
+
 	It is good practice to use a `CONFIG_` variable when assigning directory
 	names. This allows kbuild to totally skip the directory if the
 	corresponding `CONFIG_` option is neither 'y' nor 'm'.