Improve documentation of sort-related functions (#48387)
* document the `order` keyword in `sort!`
* list explicitly the required properties of `lt` in `sort!`
* clarify the sequence of "by" transformations if both `by` and `order` are given
* show default values in the signatures for `searchsorted` and related functions
* note that `by` is also applied to searched value in `searchsorted` and related
* add `isunordered` to the manual (it's already exported)
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Co-authored-by: Lilith Orion Hafner <lilithhafner@gmail.com>
