Fixed #22796 -- Added more basic explanations of forms

Now explains some more basic form handling before discussing Django;
simplifies initial examples; defers explanation of more advanced
aspects until later in the document
Having it this way means that the GET, which will be used first, is shown first. It's easier (for me anyway) to parse unfamiliar code if it's laid out that way, and the inline comments make a more sensible story. 

However given what you've said about GET and POST being the only valid methods, perhaps we could just use ``== 'GET'``.