Fix the description for aria-labelledby

[niwsa]

Reference => https://www.w3.org/TR/wai-aria-1.1/#aria-label

aria-labelledby is used when content is visible either via tooltip or visible text.

[yatil]

@niwsa I think your change is not what we want to say with this (terribly long and hard to understand) sentence. I think what we want to say is:

The aria-labelledby attribute can also be used to identify form controls. This approach is well supported by screen readers and other assistive technology. Unlike the title attribute (see below), the information is not conveyed to visual users by default. This approach should therefore only be used when the label of the control is clear from the surrounding content, like the button in the example below.

Then, we should probably add a sentence about title being not available to many users (like ones using speech input or keyboard-only).

Thanks for creating this PR. I try to make some changes soon.