Explain .panel-collapse class in docs

[aavmurphy]

It is in an "accordian" example, not a "collapse" example, which is how I missed it.

Maybe mention it in the 'collapse' section as well.

Thanks
Andrew

[Pvanhesteren]

@aavmurphy, I have just submitted #19474 to fix this. Please let me know what you think 😄

[mdo]

Bootstrap 3 is no longer being officially developed or supported.

All work has moved onto our next major release, v4. As such, this issue or pull request is being closed as a "won't fix." For additional help and support, we recommend utilizing our community resources. Thanks for your understanding, and see you on the other side of v4!

<3,
@mdo and team