Added documentation to default_ to note that it has restrictions not represented in its type (#744)

Author: LaurentRDC

beam-core/ChangeLog.md

@@ -3,12 +3,13 @@
 ## Added features
 
 * Added a `Generic` instance to `SqlNull`, `SqlBitString`, and `SqlSerial` (#736).
+* Added a note to `default_` to specify that it has more restrictions than its type may indicate (#744).
 
 # 0.10.3.1
 
 ## Updated dependencies
 
-* Updated the upper bound to include `hashable-1.5`. 
+* Updated the upper bound to include `hashable-1.5`.
 
 ## Bug fixes