doc/dev: raise prominence of logtest.Scoped, make more tweaks

Created by: bobheadxi

I have a code monitor set up for non-DevX logging usages:

repo:^github\.com/sourcegraph/sourcegraph$ type:diff log.Scoped -author:robert -author:william -author:gitstart -author:jean-hadrien patternType:literal

I see a lot of instances of folks doing log.Scoped("test", "test") in _test files. This raises the prominence of using logtest.Scoped, makes some minor docs tweaks, and also pull in an update that adds additional details on docstrings: https://github.com/sourcegraph/log/commit/adee62211aeb364f7dd438ba1ee0740e1b529513

Test plan

n/a