content: clarify that make is just one way of creating a map

Fixes golang/go#15083

Change-Id: I5c611a6bd702464d8d1afe7c7b0ca18ef077e206
Reviewed-on: https://go-review.googlesource.com/21443
Reviewed-by: Chris Broadfoot <cbro@golang.org>

content/moretypes.article

diff --git a/content/moretypes.article b/content/moretypes.article
index e314a2c..e89a962 100644
--- a/content/moretypes.article
+++ b/content/moretypes.article
@@ -194,7 +194,9 @@
 
 A map maps keys to values.
 
-Maps must be created with `make` before use; the `nil` map is empty and cannot be assigned to.
+Maps must be initialized before use; the `nil` map is empty and cannot be written to.
+
+The `make` function initializes an empty map of the given type.
 
 .play moretypes/maps.go