tags: Add the ability to make tags for all archs using "all"

make ALLSOURCE_ARCHS=all tags
- Document this in kbuild.txt
Without this change you have to type each arch separately.

Signed-off-by: John Kacur <jkacur@redhat.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>
diff --git a/Documentation/kbuild/kbuild.txt b/Documentation/kbuild/kbuild.txt
index 84725b7..634c625 100644
--- a/Documentation/kbuild/kbuild.txt
+++ b/Documentation/kbuild/kbuild.txt
@@ -162,3 +162,7 @@
 to be included in the databases, separated by blank space. E.g.:
 
     $ make ALLSOURCE_ARCHS="x86 mips arm" tags
+
+To get all available archs you can also specify all. E.g.:
+
+    $ make ALLSOURCE_ARCHS=all tags