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>

kas/includehandler.py

@@ -114,14 +114,11 @@ class IncludeHandler:
         key containing a list of includes.
 
         The includes can be specified in two ways: as a string
-        containing the relative path from the current file or as a
-        dictionary. The dictionary should have a 'file' key containing
-        the relative path to the include file and optionally a 'repo'
-        key containing the key of the repository. If the 'repo' key is
-        missing the value of the 'file' key, it is treated the same as if
-        just a string was defined, meaning the path is relative to the
-        current config file. Otherwise it is interpreted relative to
-        the repository path.
+        containing the path, relative to the repository root from the
+        current file, or as a dictionary. The dictionary must have a
+        'file' key containing the path to the include file and a 'repo'
+        key containing the key of the repository. The path is interpreted
+        relative to the repository root path.
 
         The includes are read and merged from the deepest level upwards.
     """
@@ -155,7 +152,6 @@ class IncludeHandler:
             header:
               includes:
                 - include1.yml
-                - file: include2.yml
                 - repo: repo1
                   file: include-repo1.yml
                 - repo: repo2