[docs] improve various docstrings

[odow]

Motivated by https://discourse.julialang.org/t/cpu-time-vs-wallclock/114776

https://jump.dev/JuMP.jl/previews/PR3758/api/JuMP/#solve_time

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.38%. Comparing base (fad385c) to head (15a74cc).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3758 +/- ## ======================================= Coverage 98.38% 98.38% ======================================= Files 44 44 Lines 5869 5871 +2 ======================================= + Hits 5774 5776 +2 Misses 95 95 ```

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[odow]

There are still a bunch that can be improved, but this is a good start.

[odow]

The original motivation goes from

to