doc/dev: document when soft failures are used for soc2/gn-106

Created by: bobheadxi

Closes https://github.com/sourcegraph/sourcegraph/issues/29761

The statement in the issue isn't quite correct, but I've documented our uses for soft failures here anyway, along with a sourcegraph search link 🙃 A pretty cool one at that! I did not expect this to work out so nice:

repo:^github\.com/sourcegraph/sourcegraph$ {...bk.SoftFail...} OR (...bk.SoftFail...) count:all patternType:structural

though unfortunately to get a more inclusive query that matches the entire step definition you have to define how deep the match should be, e.g. {...{...(...bk.SoftFail...)...}...} but this is good enough 😛

Also made this a search notebook: https://sourcegraph.com/notebooks/Tm90ZWJvb2s6NTc=