doc: dev process: Add a note about deprecation mechanisms

Describe how an API can be deprecated, which is via the __deprecated
keyword or by introducing a legacy Kconfig option.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>
diff --git a/doc/development_process/api_lifecycle.rst b/doc/development_process/api_lifecycle.rst
index 785d620..3bca7ed 100644
--- a/doc/development_process/api_lifecycle.rst
+++ b/doc/development_process/api_lifecycle.rst
@@ -198,7 +198,11 @@
   where an API is deprecated sooner.
 - What is required when deprecating:
 
-  - Mark as deprecated
+  - Mark as deprecated. This can be done by using the compiler itself
+    (``__deprecated`` for  function declarations and ``__DEPRECATED_MACRO`` for
+    macro definitions), or by introducing a Kconfig option (typically one that
+    contains the ``LEGACY`` word in it) that, when enabled, reverts the APIs
+    back to their previous form
   - Document the deprecation
   - Include the deprecation in the "API Changes" of the release notes for the
     next upcoming release