[Handbook]: Clarify kernel modules on upgrades

Forewarn users that FreeBSD-provided packages of kernel modules might be
unusable with a new version of the OS until some time after the version is
announced.

Approved by:		doceng@ (carlavilla@, fernape@)
Reviewed By: 		ceri@, jrm@
Differential Revision:	https://reviews.freebsd.org/D39474

documentation/content/en/books/handbook/cutting-edge/_index.adoc

@@ -245,11 +245,35 @@ Rebuilding a custom kernel, even if nothing else changed, allows `uname` to accu
 This is particularly helpful when maintaining multiple systems, as it allows for a quick assessment of the updates installed in each one.
 
 [[freebsdupdate-upgrade]]
-=== Performing Major and Minor Version Upgrades
+=== Performing Minor and Major Version Upgrades
 
-Upgrades from one minor version of FreeBSD to another, like from FreeBSD 9.0 to FreeBSD 9.1, are called _minor version_ upgrades.
-_Major version_ upgrades occur when FreeBSD is upgraded from one major version to another, like from FreeBSD 9.X to FreeBSD 10.X.
-Both types of upgrades can be performed by providing `freebsd-update` with a release version target.
+Upgrades from one minor version of FreeBSD to another are called _minor version_ upgrades. An example: 
+
+- FreeBSD 13.1 to 13.2.
+
+_Major version_ upgrades increase the major version number. An example:
+
+- FreeBSD 12.4 to 13.2.
+
+Both types of upgrade can be performed by providing `freebsd-update` with a release version target.
+
+[WARNING]
+====
+After each new `RELEASE`, FreeBSD package build servers will, for a limited period, *not* use the newer version of the operating system.
+This provides continuity for the many users who do not upgrade immediately after a release announcement.
+For example:
+
+* packages for users of 13.1 and 13.2 will be built on a server running 13.1, until 13.1 reaches end of life
+
+-- and, critically:
+
+* a kernel module that is built on 13.1 might *not* be suitable for 13.2.
+
+So, with any minor or major OS upgrade, if your package requirements include any kernel module:
+
+* *be prepared to build the module from source*.
+
+====
 
 [NOTE]
 ====
@@ -606,7 +630,7 @@ In order to track changes for the whole source tree, subscribe to {dev-commits-s
 . To install a new FreeBSD-STABLE system, install the most recent FreeBSD-STABLE release from the crossref:mirrors[mirrors,FreeBSD mirror sites] or use a monthly snapshot built from FreeBSD-STABLE. Refer to link:https://www.FreeBSD.org/snapshots/[www.freebsd.org/snapshots] for more information about snapshots.
 +
 To compile or upgrade an existing FreeBSD system to FreeBSD-STABLE, use `git` to check out the source for the desired branch.
-Branch names, such as `stable/9`, are listed at link:https://www.FreeBSD.org/releng/[www.freebsd.org/releng].
+Branch names, such as `stable/13`, are listed at link:https://www.FreeBSD.org/releng/[www.freebsd.org/releng].
 . Before compiling or upgrading to FreeBSD-STABLE , read [.filename]#/usr/src/Makefile# carefully and follow the instructions in <<makeworld>>. Read the {freebsd-stable} and [.filename]#/usr/src/UPDATING# to keep up-to-date on other bootstrapping procedures that sometimes become necessary on the road to the next release.
 
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