title: Golang RWMutex on map content edit
body: <p>I'm starting to use <code>RWMutex</code> in my Go project with <code>map</code> since now I have more than one routine running at the same time and while making all of the changes for that a doubt came to my mind.</p>
  
  <p>The thing is that I know that we must use <code>RLock</code> when only reading to allow other routines to do the same task and <code>Lock</code> when writing to full-block the map. But what are we supposed to do when editing a previously created element in the map?</p>
  
  <p>For example... Let's say I have a <code>map[int]string</code> where I do <code>Lock</code>, put inside <code>"hello "</code> and then <code>Unlock</code>. What if I want to add <code>"world"</code> to it? Should I do <code>Lock</code> or can I do <code>RLock</code>?</p>