Align include documentation to new logic

Commit 2077900b4e was wrong in claiming that the documentation already
demanded repo-relative paths for local includes. It only did that for
dictionary-based include entries. Aligning both to repo-relative paths
remain a value, but we definitely have to update the documentation now.

While at it, also update the code comments accordingly. That also still
talked about the repo key being option, something that was never the
case in practice.

Reported-by: Daniel Wagenknecht <dwagenknecht@emlix.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

docs/userguide.rst

@@ -263,7 +263,8 @@ Configuration reference
       from every included file. An item in this list can have one of two types:
 
     * item: string
-        The path to a kas configuration file, relative to the current file.
+        The path to a kas configuration file, relative to the repository root
+        of the current file.
 
     * item: dict
         If files from other repositories should be included, choose this
@@ -274,7 +275,8 @@ Configuration reference
           needs to be defined in the ``repos`` dictionary as ``<repo-id>``.
 
       * ``file``: string [required]
-          The path to the file relative to the root of the repository.
+          The path to the file, relative to the root of the specified
+          repository.
 
 * ``build_system``: string [optional]
     Defines the bitbake-based build system. Known build systems are