goprotobuf: Include package docs from .proto in the generated package docs.

These kinds of comments in .proto files usually aren't there
but if they are then they are valuable to have in the generated code.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/65860043

protoc-gen-go/generator/generator.go

@@ -311,6 +311,7 @@ func (ms *messageSymbol) GenerateAlias(g *Generator, pkg string) {
 		}
 	}
 	for _, get := range ms.getters {
+
 		if get.typeName != "" {
 			g.RecordTypeUse(get.typeName)
 		}
@@ -366,6 +367,7 @@ func (ms *messageSymbol) GenerateAlias(g *Generator, pkg string) {
 
 		g.P("func (m *", ms.sym, ") ", get.name, "() ", typ, " { return ", val, " }")
 	}
+
 }
 
 type enumSymbol string
@@ -1042,6 +1044,17 @@ func (g *Generator) generateHeader() {
 		g.P("/*")
 		g.P("Package ", name, " is a generated protocol buffer package.")
 		g.P()
+		if loc, ok := g.file.comments[strconv.Itoa(packagePath)]; ok {
+			// not using g.PrintComments because this is a /* */ comment block.
+			text := strings.TrimSuffix(loc.GetLeadingComments(), "\n")
+			for _, line := range strings.Split(text, "\n") {
+				line = strings.TrimPrefix(line, " ")
+				// ensure we don't escape from the block comment
+				line = strings.Replace(line, "*/", "* /", -1)
+				g.P(line)
+			}
+			g.P()
+		}
 		g.P("It is generated from these files:")
 		for _, f := range g.genFiles {
 			g.P("\t", f.Name)
@@ -1715,7 +1728,8 @@ func (g *Generator) generateMessage(message *Descriptor) {
 	}
 
 	if !message.group {
-		g.file.addExport(message, &messageSymbol{ccTypeName, hasExtensions, isMessageSet, getters})
+		ms := &messageSymbol{sym: ccTypeName, hasExtensions: hasExtensions, isMessageSet: isMessageSet, getters: getters}
+		g.file.addExport(message, ms)
 	}
 
 	for _, ext := range message.ext {
@@ -1926,6 +1940,7 @@ func baseName(name string) string {
 // See descriptor.proto for more information about this.
 const (
 	// tag numbers in FileDescriptorProto
+	packagePath = 2 // package
 	messagePath = 4 // message_type
 	enumPath    = 5 // enum_type
 	// tag numbers in DescriptorProto