Add some documentation describing how to interpret the symbols shown during recompilation

[pbouffard]

I started noticing the ? indicator show up in Julia 1.10 which I assume relates to parallel precompilation. I'm not exactly sure what it means nor also by what logic packages finished with precompilation (marked ✓) disappear from the list (I'm guessing that all their transitive dependencies are finished?).

This seemed to me to be the best place in the docs to put this.

[IanButterworth]

Maybe these are better as a bullet list?

Also can you add detail about yellow check marks, which indicate packages that have been precompiled, but different versions are currently loaded, so cannot be used until julia is restarted.

[IanButterworth]

bump