iter: document that calling yield after terminated range loop causes runtime panic

The existing documentation isn't clear about the behaviour of when
the iterator function ignores the value returned from the yield function.

The changes here update the documentation, to explicitly explain
that.

Change-Id: I24a8198c3da63429358554169697fa466345b8fd
GitHub-Last-Rev: 86c8a2d
GitHub-Pull-Request: #74561
Reviewed-on: https://go-review.googlesource.com/c/go/+/687215
Auto-Submit: Sean Liao <[email protected]>
Reviewed-by: Sean Liao <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Mark Freeman <[email protected]>

Author: narqo

src/iter/iter.go

@@ -28,6 +28,8 @@ or index-value pairs.
 Yield returns true if the iterator should continue with the next
 element in the sequence, false if it should stop.
 
+Yield panics if called after it returns false.
+
 For instance, [maps.Keys] returns an iterator that produces the sequence
 of keys of the map m, implemented as follows: