doc: modules: Document conventions for auto-generated Kconfig options

Document the recommended practice regarding having default definitions for
module-specific Kconfig symbols.

Signed-off-by: Johan Hedberg <johan.hedberg@silabs.com>

doc/develop/modules.rst

@@ -621,6 +621,14 @@ Zephyr modules
 In both Kconfig and CMake, the variable ``ZEPHYR_<MODULE_NAME>_MODULE_DIR``
 contains the absolute path to the module.
 
+Additionally, ``ZEPHYR_<MODULE_NAME>_MODULE`` and ``ZEPHYR_<MODULE_NAME>_MODULE_BLOBS``
+(in case the module declares blobs) symbols are automatically generated for available
+modules. These can be used e.g. to declare dependencies from other Kconfig symbols
+which depend on the module or blobs from the module. To satisfy compliance checking
+when building Zephyr without the module present, it's recommended for the module to
+have default definitions for these symbols in its respective Kconfig file under
+``modules/`` in the Zephyr main tree.
+
 In CMake, ``ZEPHYR_<MODULE_NAME>_CMAKE_DIR`` contains the
 absolute path to the directory containing the :file:`CMakeLists.txt` file that
 is included into CMake build system. This variable's value is empty if the