Rollup merge of #140075 - Urgau:midpoint-average, r=tgross35

Mention average in midpoint documentations

Added a mention to "average" in midpoint documentations and as well as some `#[doc(alias = "average")]`[^1].

This is done to improve the discoverability of the function.

[^1]: https://docs.rs/num-integer/latest/num_integer/trait.Average.html#tymethod.average_floor

Author: matthiaskrgr