maps: document handling of non-reflexive keys

Fixes #63312

Change-Id: I7861bddcd4c6dc208b4449b923eb2785a122aadf
Reviewed-on: https://go-review.googlesource.com/c/go/+/553157
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: qiu laidongfeng2 <[email protected]>
Auto-Submit: Keith Randall <[email protected]>

Author: seankhliao

src/maps/maps.go

@@ -3,6 +3,9 @@
 // license that can be found in the LICENSE file.
 
 // Package maps defines various functions useful with maps of any type.
+//
+// This package does not have any special handling for non-reflexive keys
+// (keys k where k != k), such as floating-point NaNs.
 package maps
 
 import (