Start adding (sagittarius continuations) document

Author: ktakashi

doc/sagittarius/continuations.md

@@ -1,10 +1,18 @@
 [§2] Continuations libraries {#lib.sagittarius.continuations}
 --------------------------------------------------------------
 
+##### [!Library] (sagittarius continuations) **[@since] `0.9.15`**
+
 This library provides continuation enhancement. The main capabilities
-this library provieds are below
+this library provieds below capabilities:
 
 - Continuation prompt
 - Composable continuation
 - Delimited continuation
 
+
+###### [!Function] `continuation?` _obj_
+
+Returns `#t` if the given _obj_ is a continuation object, otherwise `#f`.
+
+NOTE: a continuation object is also a procedure.