stubgen: docs for llndk-deprecate

This doesn't do anything now, but we are using it
to document an API used to be in the LL-NDK. This
is important so that we know not to remove the
symbol (e.g. if it is only in the LL-NDK and not
in the NDK).

Bug: 317293882
Test: N/A
Change-Id: If67715be5572ade7ce1e7c3bd9bc7a98f3da39f5
diff --git a/docs/map_files.md b/docs/map_files.md
index 35e8cbb..37f91ec 100644
--- a/docs/map_files.md
+++ b/docs/map_files.md
@@ -134,6 +134,9 @@
 
 Historically this annotation was spelled `vndk`, but it has always meant LL-NDK.
 
+When an llndk API is deprecated, the `llndk` tag is dropped and
+`llndk-deprecate=<V>` is added.
+
 ### platform-only
 
 Indicates that the version or symbol is public in the implementation library but