Add general cross-reference for low/high-level crypto

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/docs/3.0-migration-guide.md b/docs/3.0-migration-guide.md
index 04eb37f..35652c3 100644
--- a/docs/3.0-migration-guide.md
+++ b/docs/3.0-migration-guide.md
@@ -134,6 +134,9 @@
 
 ## Low-level crypto
 
+Please also refer to the section [High-level crypto](#high-level-crypto) for
+changes that could sit in either cateogry.
+
 ### The RNG parameter is now mandatory for all functions that accept one
 
 This change affects all users who called a function accepting a `f_rng`
@@ -240,6 +243,9 @@
 
 ## High-level crypto
 
+Please also refer to the section [Low-level crypto](#low-level-crypto) for
+changes that could sit in either cateogry.
+
 ### Remove wrapper for libpkcs11-helper
 
 This doesn't affect people using the default configuration as it was already