dev/ref-manual: Document INIT_MANAGER

The INIT_MANAGER variable was added in 3.0 but it seems we didn't get
around to documenting it yet. I have added a variable glossary entry and
made the basic adjustment of the "Using systemd Exclusively" section in
the dev manual, however I think the latter section still needs work.

(From yocto-docs rev: a84adfaeda5a790275d020fc77d721e8560a5728)

Signed-off-by: Paul Eggleton <bluelightning@bluelightning.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

documentation/ref-manual/variables.rst

@@ -3701,6 +3701,21 @@ system and gives an overview of their function and contents.
          even if the toolchain's binaries are strippable, there are other files
          needed for the build that are not strippable.
 
+   :term:`INIT_MANAGER`
+      Specifies the system init manager to use. Available options are:
+
+      -  ``sysvinit`` - System V init (default for poky)
+      -  ``systemd`` - systemd
+      -  ``mdev-busybox`` - mdev provided by busybox
+      -  ``none`` - no init manager
+
+      More concretely, this is used to include
+      ``conf/distro/include/init-manager-${INIT_MANAGER}.inc`` into the global
+      configuration. You can have a look at the ``conf/distro/include/init-manager-*.inc``
+      files for more information, and also the
+      ":ref:`dev-manual/init-manager:selecting an initialization manager`"
+      section in the Yocto Project Development Tasks Manual.
+
    :term:`INITRAMFS_DEPLOY_DIR_IMAGE`
       Indicates the deploy directory used by ``do_bundle_initramfs`` where the
       :term:`INITRAMFS_IMAGE` will be fetched from.