Clarify any extra arguments on boot-constant dispatcher

docs/dispatchers.md

@@ -30,12 +30,16 @@ immutable, thus providing the optimizer with options.
 
 ### boot-constant
 ```
-nqp::dispatch('boot-constant', $value);
+nqp::dispatch('boot-constant', $value, …);
 ```
 Take the first argument and produce it as the result, but also treat it as
 a constant value that will always be produced (thus meaning the optimizer
 could consider any pure code used to calculate the value as dead).
 
+Ignores all other arguments: this allows calls to this dispatcher to be
+using an existing capture by just inserting a value as the first argument,
+thus removing the need to remove any superfluous arguments.
+
 ### boot-resume
 ```
 nqp::dispatch('boot-resume');