Make {.reorder: on.} an {.experimental.} pragma and add to documentation

[awr1]

Self-explanatory. Issues like #8665 show {.reorder: on.} is not quite ready for primetime.

[Clyybber]

#8665 is about to be fixed by #8692 and there dont seem to be any issues with reorder except for that issue.

[awr1]

If anything the lack of docs suggest that not enough people have been giving it a look to help ascertain its stability. It seems that it was introduced late last year - how long does it usually take for a feature to lose the experimental pragma?

[dom96]

You already need to enable reorder explicitly. It might make sense to change {.reorder: on.} to {experimental: "reorder".} to make it clearer that this is experimental, but if this wasn't an experimental feature it would be enabled by default.