[PATCH v3 5/6] doc: Update docs to fit new IMAGE_TYPE meaning

[Uladzimir Bely <ubely@ilbers.de>]

While now IMAGE_TYPE allows to specify several image types,
separated by space, this needs to be mentioned in manuals.

Signed-off-by: Uladzimir Bely <ubely@ilbers.de>
---
 doc/user_manual.md | 17 ++++++++++++++---
 1 file changed, 14 insertions(+), 3 deletions(-)

diff --git a/doc/user_manual.md b/doc/user_manual.md
index 535d456a..f8f58a5c 100644
--- a/doc/user_manual.md
+++ b/doc/user_manual.md
@@ -210,7 +210,10 @@ tmp/deploy/images/rpi/isar-image-base.rpi-sdimg
 
 ### Generate full disk image
 
-A bootable disk image is generated if you set IMAGE_TYPE to 'wic-img'. Behind the scenes a tool called `wic` is used to assemble the images. It is controlled by a `.wks` file which you can choose with changing WKS_FILE. Some examples in the tree use that feature already.
+A bootable disk image is generated if `wic-img` is listed in IMAGE_TYPE.
+Behind the scenes a tool called `wic` is used to assemble the images.
+It is controlled by a `.wks` file which you can choose with changing WKS_FILE.
+Some examples in the tree use that feature already.
 ```
  # Generate an image for the `i386` target architecture
  $ bitbake mc:qemui386-buster:isar-image-base
@@ -246,7 +249,7 @@ https://github.com/intel/bmap-tools
 
 ### Generate container image with root filesystem
 
-A runnable container image is generated if you set IMAGE_TYPE to 
+A runnable container image is generated if IMAGE_TYPE variable includes
 'container-img'.
 Getting a container image can be the main purpose of an Isar configuration, 
 but not only.
@@ -457,6 +460,14 @@ Isar can generate various images types for specific machine. The type of the ima
  - `ubi-img` - A image for use on mtd nand partitions employing UBI
  - `vm-img` - A image for use on VirtualBox or VMware
 
+There are several image types can be listed in `IMAGE_TYPE` divided by space.
+
+Instead of setting multiple image types in one target, user can also use
+[multiconfig](#building-target-images-for-multiple-configurations) feature and specify
+different image types in different multiconfigs (use qemuamd64-buster-cpiogz.conf
+and qemuamd64-buster-tgz.conf as examples). The only requirement is that image types
+from different multiconfigs for the same machine/distros should not overlap.
+
 ---
 
 ## Add a New Distro
@@ -508,7 +519,7 @@ Every machine is described in its configuration file. The file defines the follo
  - `KERNEL_IMAGE` - The name of kernel binary that it installed to `/boot` folder in target filesystem. This variable is used by Isar to extract the kernel binary and put it into the deploy folder. This makes sense for embedded devices, where kernel and root filesystem are written to different flash partitions. This variable is optional.
  - `INITRD_IMAGE` - The name of `ramdisk` binary. The meaning of this variable is similar to `KERNEL_IMAGE`. This variable is optional.
  - `MACHINE_SERIAL` - The name of serial device that will be used for console output.
- - `IMAGE_TYPE` - The type of images to be generated for this machine.
+ - `IMAGE_TYPE` - The types of images to be generated for this machine.
 
 Below is an example of machine configuration file for `Raspberry Pi` board:
 ```
-- 
2.20.1