Make fromCharCodeArray slightly less formal

This avoids having to refer to actual Wasm instructions,
since after all this is a host function.

Author: ajklein

document/js-api/index.bs

@@ -1851,21 +1851,26 @@ When this builtin is invoked with parameter |v|, the following steps must be run
 
 The |funcType| of this builtin is `(func (param (ref null (array (mut i16))) i32 i32) (result externref))`.
 
+Note: This function only takes a mutable i16 array defined in its own recursion group.
+If this is an issue for toolchains, we can look into how to relax the function type
+while still maintaining good performance.
+
 <div algorithm="js-string-fromCharCodeArray">
 When this builtin is invoked with parameters |array|, |start|, and |end|, the following steps must be run:
 
-1. If |array| is nul]
+1. If |array| is null
     1. Throw a {{RuntimeError}} exception. TODO: this needs to not be catchable, like a trap.
-1. If |start| > |end| or |end| > [=array_len=](|array|)
+1. Let |length| be the number of elements in |array|.
+1. If |start| > |end| or |end| > |length|
     1. Throw a {{RuntimeError}} exception. TODO: this needs to not be catchable, like a trap.
-1. Let |result| be the empty string
-1. Let |i| be |start|
+1. Let |result| be the empty string.
+1. Let |i| be |start|.
 1. While |i| < |end|:
-    1. Let |charCode| be [=array_i16_get=](|array|, |i|).
+    1. Let |charCode| be the value of the element stored at index |i| in |array|.
     1. Let |charCodeString| be [$FromCharCode$](|charCode|).
     1. Let |result| be the concatenation of |result| and |charCodeString|.
     1. Set |i| to |i| + 1.
-1. Return |result|
+1. Return |result|.
 
 </div>