Emphasize use of required build tag. (#1073)

Author: kennwhite

mongo/doc.go

@@ -102,7 +102,9 @@
 // # Client Side Encryption
 //
 // Client-side encryption is a new feature in MongoDB 4.2 that allows specific data fields to be encrypted. Using this
-// feature requires specifying the "cse" build tag during compilation.
+// feature requires specifying the "cse" build tag during compilation:
+//
+//	go build -tags cse
 //
 // Note: Auto encryption is an enterprise-only feature.
 //