Improvements to stateful button JS docs

[cvrebert]

• Mark stateful button feature as deprecated.
• Document that $(...).button('reset') is async; addresses Cannot disable stateful button #16362.

[HEdingfield]

Sorry to ask here, but I couldn't find the info anywhere else: why is the stateful button being deprecated and what's it being replaced with? I use these pretty extensively...

[cvrebert]

See the thread mentioned in the commit message (#16362 (comment)), but to add some more detail:
The feature doesn't do very much, there are some reasonable edge cases that don't seem possible to satisfy simultaneously (#16362, #14450), and it complicates the implementation of the main checkboxes/radios part of the buttons plugin. It will not have a replacement; it's being straight-up removed in Bootstrap v4. We recommend writing your own tiny trivial snippet of JavaScript instead.

For the avoidance of doubt: We intend to retain the rest of the buttons plugin (checkboxes, radios, single-toggle) in v4.

[scottdorman]

@cvrebert With this piece of the buttons plugin being removed in v4, are there any recommendations for a replacement that we can use now instead of using what's currently in v3?

[cvrebert]

We recommend writing your own tiny trivial snippet of JavaScript instead.