kbuild: clarify the difference between obj-y and obj-m w.r.t. descending

Kbuild descends into a directory by either 'y' or 'm', but there is an
important difference.

Kbuild combines the built-in objects into built-in.a in each directory.
The built-in.a in the directory visited by obj-y is merged into the
built-in.a in the parent directory. This merge happens recursively
when Kbuild is ascending back towards the top directory, then built-in
objects are linked into vmlinux eventually. This works properly only
when the Makefile specifying obj-y is reachable by the chain of obj-y.

On the other hand, Kbuild does not take built-in.a from the directory
visited by obj-m. This it, all the objects in that directory are
supposed to be modular. If Kbuild descends into a directory by obj-m,
but the Makefile in the sub-directory specifies obj-y, those objects
are just left orphan.

The current statement "Kbuild only uses this information to decide that
it needs to visit the directory" is misleading. Clarify the difference.

Reported-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Johan Hovold <johan@kernel.org>

Documentation/kbuild/makefiles.rst

@@ -297,9 +297,19 @@ more details, with real examples.
 	If CONFIG_EXT2_FS is set to either 'y' (built-in) or 'm' (modular)
 	the corresponding obj- variable will be set, and kbuild will descend
 	down in the ext2 directory.
-	Kbuild only uses this information to decide that it needs to visit
-	the directory, it is the Makefile in the subdirectory that
-	specifies what is modular and what is built-in.
+
+	Kbuild uses this information not only to decide that it needs to visit
+	the directory, but also to decide whether or not to link objects from
+	the directory into vmlinux.
+
+	When Kbuild descends into the directory with 'y', all built-in objects
+	from that directory are combined into the built-in.a, which will be
+	eventually linked into vmlinux.
+
+	When Kbuild descends into the directory with 'm', in contrast, nothing
+	from that directory will be linked into vmlinux. If the Makefile in
+	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.
 
 	It is good practice to use a `CONFIG_` variable when assigning directory
 	names. This allows kbuild to totally skip the directory if the