We've made changes to the diagnostic verifier over time that we failed to actually document. Update docs/Diagnostics.md to describe:
* Fix-it alternation (cdcd726f92)
* Fix-it line offsets (2128678d56)
* Additional files (5036a55550)
* Ignoring `<unknown>` diagnostics (384ab780e9)
I've also slightly fleshed out the description of fix-it expectations.
* [Diagnostics] Turn educational notes on-by-default
* [Diagnostics] Only include educational notes in printed output if -print-educational-notes is passed
* Make -print-educational-notes a driver option
* [Diagnostics] Issue a printed remark if educational notes are available, but disabled
* [docs] Update educational notes documentation and add a contributing guide
* [Diagnostics] Cleanup PrintingDiagnosticConsumer handling of edu notes
* Revert "[Diagnostics] Issue a printed remark if educational notes are available, but disabled"
For now, don't notify users if edu notes are available but disabled. This decision can be reevaluated later.
A handful of people have recently asked me if such a thing existed;
now it does. There's certainly more that can be added here, but I
wanted to dump a bunch of the information in my brain into doc form.
