Improvements to asynchronous HTTP function pages

[YYBartT]

The manual's Asynchronous HTTP Functions page currently lists all functions without providing a basic explanation of HTTP requests. It also lacks an overview on which function to use when. http_request() is the most "generic" function that can be used and http_get(), http_get_file() and http_post_string() can be used in more specific cases.

To be changed:

• Add a short introduction to the asynchronous HTTP functions page and point out which function to use where/when. Link this introduction to a basic explanation of HTTP GET and POST requests (i.e. HTTP requests have a header and a body, body is optional, etc.).
• Add a complete code example on how to handle an HTTP request (or, alternatively, only a basic GET), complete with code in the async HTTP event that handles the entire request from start to end (i.e. from async_load[? "status"] equal to 1 up to the last event where it equals 0). Also add some code to the example that shows the progress bar.
• Make a clear distinction between URL parameters and request body.
• Possibly a few other things.

Brought up while doing changes for: YoYoGames/GameMaker-Bugs#5744

[Emc1923]

Verified changes in the monthly manual, closing.