[docs] Forms Documentation Improvement

[dashtinejad]

I feel that the forms documentation is missing some important and fundamentals inputs:

• multi select
• checkbox
• radio buttons

So, I've added them to the documentation to clarify how to convert those form inputs to React Controlled Components.

[lacker]

"multiselect" isn't really its own thing - it's just a feature of select. How about making this example inline with the previous section, and just saying something like:

"You can also do multiple selection, by setting the 'multiple' parameter and using an array as the value."

...followed by sample code that just changes the example to a multiselect example

[lacker]

I think it would be better to combine the JSX and javascript code in this example. Look at how textarea works, the format is just:

--
In HTML a textarea is like this:

(example)

In React, you change X and Y, so a full example is this:

(full example)

Note some brief stuff stuff.

How about using that parallel format for checkbox and radio buttons?

[lacker]

Originally I was thinking, meh we don't need these examples, but by now a number of folks have suggested re-adding them so I officially change my mind. So it would be cool to add these examples, I just suggest using a format equivalent to the way "textarea" works where the example is a full one rather than describing it bit by bit - I added comments to that effect inline

[dashtinejad]

@lacker Good points, I'll work on it.

[gaearon]

@keyanzhang Would you like to help reviewing this? Since you worked on a related section recently in another PR.

[lacker]

AFAICT this is still in progress rather than needing review so I tagged it back. Let me know if that is not the case though

[dashtinejad]

@lacker As you suggested, I've added 2 new sections (for checkbox and radio) using a format equivalent to the way "textarea" where the example is a full one. Also I've added a note about multi attribute for select tag. Please let me know if I missed something.

[sebmarkbage]

Changes requested above.

[dashtinejad]

@lacker @gaearon @sebmarkbage I really do believe that handling checkbox and radio elements should be on official documentation. Please let me know If I should improve my suggested documentation.

[bvaughn]

Thank you for submitting this PR! 😄

Unfortunately the documentation and source code for reactjs.org has been moved to a different repository: https://github.com/reactjs/reactjs.org

I realize this PR is old and there may no longer be interest in seeing it merged. If there is, please open a new PR there. Sorry for the inconvenience!

(For more backstory on why we moved to a new repo, see issue #11075)