Add various references to native histograms

[beorn7]

This is not yet an exhaustive documentation. The changes here are mostly meant to avoid confusion.

Once native histograms are closer to becoming a stable feature, we will be able (and should) provide more detailed documentation.

Note that additional documentation is provided as part of the v2.40 release in the prometheus/prometheus repository.

(We should probably hold back merging this until at least a RC of v2.40 is available.)

[juliusv]

:+1: besides nits

[beorn7]

Nits addressed.

I'll hold back merging this until https://github.com/prometheus/prometheus/pull/11447 is merged.