Improve security section

[mtrezza]

Based on https://github.com/parse-community/docs/pull/821, this PR

• moves the added security note regarding master key into the existing master key section
• refactors the section as Security section to give it more prominence

[mtrezza]

@dblythy What do you think?

[dblythy]

I think this an improvement. I think this is a good edition as I think this concern is probably relevant to returning objects from cloud functions, such as:

Parse.Cloud.define('getStats', async () => {
  const stat = await new Parse.Query('SecureStat').first({useMasterKey:true});
  return stat; // insecure returns whole object (I think)
  return stat.get('count') // only return relevant fields
});

I like the idea of a security section and think it will continue to grow as we continue to evolve the docs to recommend best practice guidelines.