Add access modification behavior to compatibility-faq.md
Bug: b/200087509
Change-Id: I839669aedb56fa6b36f47f5c941d10163632f9f6
diff --git a/compatibility-faq.md b/compatibility-faq.md
index 37109c4..00b98a5 100644
--- a/compatibility-faq.md
+++ b/compatibility-faq.md
@@ -33,6 +33,10 @@
rewritten to `SourceFile` unless `-renamesourcefileattribute` is used in which
case the provided value is used. The original source file name is in the mapping
file and when optimizing or minifying a mapping file is always produced.
+- Access modification is enabled by default when optimizations are enabled
+(`-allowaccessmodification`). When compiling a library to class files, access
+modification will protect the API surface of the library by *not* changing the
+visibility of fields and methods on kept classes and their super classes.
## Stack traces and retracing
When compiling with R8, a mapping file can be produced to support mapping stack
