docs/manual: promote using fixed version for kernel headers when contributing a board

When the default (newest) kernel headers series changes the build can
break. Example error message:

  Incorrect selection of kernel headers: expected 6.8.x, got 6.5.x

In the above case the defconfig used:

  BR2_LINUX_KERNEL_CUSTOM_VERSION=y
  BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="6.5.9"

The kernel headers were not specified, so the build defaulted to using
the kernel sources as header source and the default (newest) header
series. From .config:

  BR2_KERNEL_HEADERS_AS_KERNEL=y
  BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_6_8=y

Signed-off-by: Gero Schwäricke <gero.schwaericke@posteo.de>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

docs/manual/adding-board-support.adoc

@@ -32,7 +32,11 @@ Always use fixed versions or commit hashes for the different
 components, not the "latest" version. For example, set
 +BR2_LINUX_KERNEL_CUSTOM_VERSION=y+ and
 +BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE+ to the kernel version you tested
-with.
+with. If you are using the buildroot toolchain +BR2_TOOLCHAIN_BUILDROOT+
+(which is the default), additionally ensure that the same kernel headers
+are used (+BR2_KERNEL_HEADERS_AS_KERNEL+, which is also the default) and
+set the custom kernel headers series to match your kernel version
+(+BR2_PACKAGE_HOST_LINUX_HEADERS_CUSTOM_*+).
 
 It is recommended to use as much as possible upstream versions of the
 Linux kernel and bootloaders, and to use as much as possible default