docs: update HOWTO for 3.x -> 4.x versioning

The HOWTO document needed updating for the new kernel versioning.
Signed-off-by: Mario Carrillo <mario.alfredo.c.arevalo@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/HOWTO

@@ -218,16 +218,16 @@ The development process
 Linux kernel development process currently consists of a few different
 main kernel "branches" and lots of different subsystem-specific kernel
 branches.  These different branches are:
-  - main 3.x kernel tree
-  - 3.x.y -stable kernel tree
-  - 3.x -git kernel patches
+  - main 4.x kernel tree
+  - 4.x.y -stable kernel tree
+  - 4.x -git kernel patches
   - subsystem specific kernel trees and patches
-  - the 3.x -next kernel tree for integration tests
+  - the 4.x -next kernel tree for integration tests
 
-3.x kernel tree
+4.x kernel tree
 -----------------
-3.x kernels are maintained by Linus Torvalds, and can be found on
-kernel.org in the pub/linux/kernel/v3.x/ directory.  Its development
+4.x kernels are maintained by Linus Torvalds, and can be found on
+kernel.org in the pub/linux/kernel/v4.x/ directory.  Its development
 process is as follows:
   - As soon as a new kernel is released a two weeks window is open,
     during this period of time maintainers can submit big diffs to
@@ -262,20 +262,20 @@ mailing list about kernel releases:
 	released according to perceived bug status, not according to a
 	preconceived timeline."
 
-3.x.y -stable kernel tree
+4.x.y -stable kernel tree
 ---------------------------
 Kernels with 3-part versions are -stable kernels. They contain
 relatively small and critical fixes for security problems or significant
-regressions discovered in a given 3.x kernel.
+regressions discovered in a given 4.x kernel.
 
 This is the recommended branch for users who want the most recent stable
 kernel and are not interested in helping test development/experimental
 versions.
 
-If no 3.x.y kernel is available, then the highest numbered 3.x
+If no 4.x.y kernel is available, then the highest numbered 4.x
 kernel is the current stable kernel.
 
-3.x.y are maintained by the "stable" team <stable@vger.kernel.org>, and
+4.x.y are maintained by the "stable" team <stable@vger.kernel.org>, and
 are released as needs dictate.  The normal release period is approximately
 two weeks, but it can be longer if there are no pressing problems.  A
 security-related problem, instead, can cause a release to happen almost
@@ -285,7 +285,7 @@ The file Documentation/stable_kernel_rules.txt in the kernel tree
 documents what kinds of changes are acceptable for the -stable tree, and
 how the release process works.
 
-3.x -git patches
+4.x -git patches
 ------------------
 These are daily snapshots of Linus' kernel tree which are managed in a
 git repository (hence the name.) These patches are usually released
@@ -317,9 +317,9 @@ revisions to it, and maintainers can mark patches as under review,
 accepted, or rejected.  Most of these patchwork sites are listed at
 http://patchwork.kernel.org/.
 
-3.x -next kernel tree for integration tests
+4.x -next kernel tree for integration tests
 ---------------------------------------------
-Before updates from subsystem trees are merged into the mainline 3.x
+Before updates from subsystem trees are merged into the mainline 4.x
 tree, they need to be integration-tested.  For this purpose, a special
 testing repository exists into which virtually all subsystem trees are
 pulled on an almost daily basis: