Add a paragraph about pack iteration

Cherry-picked from #206

Author: simanerush

TSPL.docc/ReferenceManual/Statements.md

@@ -70,19 +70,19 @@ and a `continue` statement and is discussed in <doc:Statements#Break-Statement>
 A `for`-`in` statement allows a block of code to be executed
 once for each item in a collection (or any type)
 that conforms to the
-[`Sequence`](https://developer.apple.com/documentation/swift/sequence) protocol.
+[`Sequence`](https://developer.apple.com/documentation/swift/sequence) protocol, or in a value parameter pack.
 
 A `for`-`in` statement has the following form:
 
 ```swift
-for <#item#> in <#collection#> {
+for <#item#> in <#expression#> {
    <#statements#>
 }
 ```
 
-The `makeIterator()` method is called on the *collection* expression
-to obtain a value of an iterator type --- that is,
-a type that conforms to the
+If the *expression* of a `for`-`in` statement is a collection *expression*, the
+`makeIterator()` method is called on it to obtain a value of an iterator type
+ --- that is, a type that conforms to the 
 [`IteratorProtocol`](https://developer.apple.com/documentation/swift/iteratorprotocol) protocol.
 The program begins executing a loop
 by calling the `next()` method on the iterator.
@@ -93,6 +93,16 @@ and then continues execution at the beginning of the loop.
 Otherwise, the program doesn't perform assignment or execute the *statements*,
 and it's finished executing the `for`-`in` statement.
 
+The *expression* of a `for`-`in` statement may also be a pack expansion 
+*expression*. In this case, on the *i*th iteration, the type of *item* is the 
+*i*th type parameter in the type parameter pack being iterated over. The value 
+of *item* is the pattern type of *expression*, with each captured type 
+parameter pack replaced with an implicit scalar type parameter with matching
+requirements. The *expression* is evaluated once at each iteration, instead of
+`n` times eagerly, where `n` is the length of the packs captured by the 
+pattern. The program will continue executing *statements* while the total 
+length of the packs captured by the pattern isn't reached.
+
 > Grammar of a for-in statement:
 >
 > *for-in-statement* → **`for`** **`case`**_?_ *pattern* **`in`** *expression* *where-clause*_?_ *code-block*