Add comment with HTTPS example

[weiying-chen]

This PR will hopefully prevent this from happening again.

At first, I thought about creating a separate https_client.rs, but that would create a lot of duplicate code (also it'd look strange if http_client.rs had an HTTPS version and http_server.rs didn't). I feel the comment will get the message across with no code duplication.

Note: There's aliasing here:

use esp_idf_svc::http::client::{Configuration as HttpConfiguration, EspHttpConnection};

To prevent conflict with this:

use embedded_svc::{
    // More imports
    wifi::{AuthMethod, ClientConfiguration, Configuration},
};