In particular, internal/abi now has some actual code,
so obfuscating those literals was breaking as expected.
Document how to update the list in the future as well.
The change above gets "go test" to just one test failure on:
go version devel go1.18-578ada410d Tue Nov 9 22:58:24 2021 +0000 linux/amd64
We also move the doc about why we disable GarbleLiterals,
so that it's next to where the disabling happens.
While here, we also rename GarbleLiterals to ObfuscateLiterals,
as we have been trying to move away from "to garble" as a verb.
Finally, limit the verbosity of diffoscope.
One test was failing for me, and diffoscope printed thousands of lines.
Not particularly useful when I'm trying to skim test results.
Usually, seeing a few dozen lines of output is enough.
Updates #385.