[doc] Extend documentation on align-content

[maurer2]

Hello, align-content for block and table elements recently gained support in all main browsers (see caniuse for block elements), so it can potentially also be used to vertical align child elements in said elements. I prepared an example here: https://play.tailwindcss.com/RSyYxxieln

At the moment the documentation on align-content)[content] only mentions grid and flexbox, but it might make sense to mention, that most modern browsers now also support this property for the aforementioned elements as well.

Cheers