private/model/api: Add custom doc blob doc string

Adds custom doc string to blob types to document the fact the SDK will
automatically encode and decode the value as it goes across the wire.
Presenting the user with a raw binary []byte type. Without needing to
manually encode/decode the value.

Fix aws#559

private/model/api/customization_passes.go

@@ -1,6 +1,7 @@
 package api
 
 import (
+	"fmt"
 	"path/filepath"
 	"strings"
 )
@@ -16,6 +17,34 @@ func (a *API) customizationPasses() {
 	if fn := svcCustomizations[a.PackageName()]; fn != nil {
 		fn(a)
 	}
+
+	blobDocStringCustomizations(a)
+}
+
+const base64MarshalDocStr = "// %s is automatically base64 encoded/decoded by the SDK.\n"
+
+func blobDocStringCustomizations(a *API) {
+	for _, s := range a.Shapes {
+		payloadMemberName := s.Payload
+
+		for refName, ref := range s.MemberRefs {
+			if refName == payloadMemberName {
+				// Payload members have their own encoding and may
+				// be raw bytes or io.Reader
+				continue
+			}
+			if ref.Shape.Type == "blob" {
+				docStr := fmt.Sprintf(base64MarshalDocStr, refName)
+				if len(strings.TrimSpace(ref.Shape.Documentation)) != 0 {
+					ref.Shape.Documentation += "//\n" + docStr
+				} else if len(strings.TrimSpace(ref.Documentation)) != 0 {
+					ref.Documentation += "//\n" + docStr
+				} else {
+					ref.Documentation = docStr
+				}
+			}
+		}
+	}
 }
 
 // s3Customizations customizes the API generation to replace values specific to S3.