Update adaptor documentation

[tcbrindle]

• Make sure each adaptor has at least a minimal explanation of what it does
• For each function, add a table detailing which concepts the returned adaptor supports and under what circumstances
• Add a bit of introductory blurb explaining about argument passing and warning about using flux::ref

There's still more to do - many of the adaptors still need usage examples, "see also" links and other things, but this is better than it was.

[codecov[bot]]

Codecov Report

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

Comparison is base (64f0335) 97.99% compared to head (f512140) 97.99%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #156 +/- ## ======================================= Coverage 97.99% 97.99% ======================================= Files 66 66 Lines 2392 2392 ======================================= Hits 2344 2344 Misses 48 48 ```

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