dt-bindings: writing-schema: add example for multiple DT_SCHEMA_FILES

Variable DT_SCHEMA_FILES can specify multiple files or patterns, but the
documentation does not indicate how to do it. Each entry needs to be
separated by a column symbol ':'.

Add an example to make it more obvious how to specify multiple
DT_SCHEMA_FILES.
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>
Link: https://lore.kernel.org/r/20231002183813.1702052-1-hugo@hugovil.comSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/writing-schema.rst

@@ -159,11 +159,14 @@ It is possible to run both in a single command::
     make dt_binding_check dtbs_check
 
 It is also possible to run checks with a subset of matching schema files by
-setting the ``DT_SCHEMA_FILES`` variable to a specific schema file or pattern.
+setting the ``DT_SCHEMA_FILES`` variable to 1 or more specific schema files or
+patterns (partial match of a fixed string). Each file or pattern should be
+separated by ':'.
 
 ::
 
     make dt_binding_check DT_SCHEMA_FILES=trivial-devices.yaml
+    make dt_binding_check DT_SCHEMA_FILES=trivial-devices.yaml:rtc.yaml
     make dt_binding_check DT_SCHEMA_FILES=/gpio/
     make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml