docs: Example for integration-specific context args. (#3455)

* Update authentication.md

I follow this example on Azure Function and it failed. After a little digging, I found out the req object is passed as "request". There may be some inconsistency in implementation.

* Adjust #3455 to account for new content in #3506.

This should hopefully make it more clear what information is available on
the argument to `context`, and how it might need to be adjusted for each
specific integration.

Co-authored-by: Jesse Rosenberger <git@jro.cc>

docs/source/security/authentication.md

@@ -23,12 +23,20 @@ const server = new ApolloServer({
  typeDefs,
  resolvers,
  context: ({ req }) => {
-   // get the user token from the headers
+   // Note! This example uses the `req` object to access headers,
+   // but the arguments received by `context` vary by integration.
+   // This means they will vary for Express, Koa, Lambda, etc.!
+   //
+   // To find out the correct arguments for a specific integration,
+   // see the `context` option in the API reference for `apollo-server`:
+   // https://www.apollographql.com/docs/apollo-server/api/apollo-server/
+
+   // Get the user token from the headers.
    const token = req.headers.authorization || '';
-  
+
    // try to retrieve a user with the token
    const user = getUser(token);
-  
+
    // add the user to the context
    return { user };
  },