kbuild: Update documentation of clean-files and clean-dirs

Commit a16c5f99 (kbuild: Fix removal of the debian/ directory) slightly
changed the processing of the clean-files and clean-dirs variables.
Also, use a current real-world example of clean-files usage.

Signed-off-by: Michal Marek <mmarek@suse.cz>

Documentation/kbuild/makefiles.txt

@@ -751,12 +751,12 @@ generated by kbuild are deleted all over the kernel src tree when
 Additional files can be specified in kbuild makefiles by use of $(clean-files).
 
 	Example:
-		#drivers/pci/Makefile
+		#lib/Makefile
-		clean-files := devlist.h classlist.h
+		clean-files := crc32table.h
 
 When executing "make clean", the two files "devlist.h classlist.h" will be
 deleted. Kbuild will assume files to be in the same relative directory as the
-Makefile except if an absolute path is specified (path starting with '/').
+Makefile, except if prefixed with $(objtree).
 
 To delete a directory hierarchy use:
 
@@ -764,9 +764,8 @@ To delete a directory hierarchy use:
 		#scripts/package/Makefile
 		clean-dirs := $(objtree)/debian/
 
-This will delete the directory debian, including all subdirectories.
+This will delete the directory debian in the toplevel directory, including all
-Kbuild will assume the directories to be in the same relative path as the
+subdirectories.
-Makefile if no absolute path is specified (path does not start with '/').
 
 To exclude certain files from make clean, use the $(no-clean-files) variable.
 This is only a special case used in the top level Kbuild file: