Add vector function changes to 7.4 migration docs. (#46893)

This note was accidentally omitted in the breaking change #46004.

Author: jtibshirani

docs/reference/migration/migrate_7_4.asciidoc

@@ -39,6 +39,12 @@ If a document doesn't have a value for a vector field (dense_vector
 or sparse_vector) on which a vector function is executed, an error will
 be thrown.
 
+[discrete]
+==== Use float instead of double for query vectors
+Previously, vector functions like `cosineSimilarity` represented the query
+vector as an list of doubles. Now vector functions use floats, which matches
+how the stored document vectors are represented.
+
 [discrete]
 [[breaking_74_snapshots_changes]]
 === Snapshot and Restore changes