stable: Update documentation to clarify preferred procedure

Clearly specify that "Cc: stable@vger.kernel.org" is strongly preferred so
that developers understand that the other options should only be used when
absolutely required.

Also specify how upstream commit ids should be referenced in patches
submitted directly to stable (I gathered this from looking at the stable
archives), and specify that any modified patches for stable should be
clearly documented in the patch description.
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/stable_kernel_rules.txt

@@ -59,11 +59,20 @@ For all other submissions, choose one of the following procedures:
    changelog of your submission, as well as the kernel version you wish
    it to be applied to.
 
-Option 1 is probably the easiest and most common. Options 2 and 3 are more
-useful if the patch isn't deemed worthy at the time it is applied to a public
-git tree (for instance, because it deserves more regression testing first).
-Option 3 is especially useful if the patch needs some special handling to apply
-to an older kernel (e.g., if API's have changed in the meantime).
+Option 1 is *strongly* preferred, is the easiest and most common.  Options 2 and
+3 are more useful if the patch isn't deemed worthy at the time it is applied to
+a public git tree (for instance, because it deserves more regression testing
+first).  Option 3 is especially useful if the patch needs some special handling
+to apply to an older kernel (e.g., if API's have changed in the meantime).
+
+Note that for Option 3, if the patch deviates from the original upstream patch
+(for example because it had to be backported) this must be very clearly
+documented and justified in the patch description.
+
+The upstream commit ID must be specified with a separate line above the commit
+text, like this:
+
+    commit <sha1> upstream.
 
 Additionally, some patches submitted via Option 1 may have additional patch
 prerequisites which can be cherry-picked. This can be specified in the following