documentation: Adds package level comments (#2612)

* ADDED package level comments for: benchmark/, examples/helloworld/

* UPDATED code to run gofmt

* FIXED linter errors

* FIXED spelling error

* FIXED documentation based on feedback

* FIXED comment to add pointer to another file

* ADDED reference to start server

* ADDED fix to include command to start server

examples/helloworld/greeter_server/main.go

@@ -18,6 +18,7 @@
 
 //go:generate protoc -I ../helloworld --go_out=plugins=grpc:../helloworld ../helloworld/helloworld.proto
 
+// Package main implements a server for Greeter service.
 package main
 
 import (