I have just started to read the PostgreSQL code and found a lack of comments for a postgres backend program in bootstrap mode. When I saw the --boot option implemented in src/backend/main/main.c at first time, I did not understand why the --boot option is not documented and what it is used for. The only way to know these things is to type `grep -r '\--boot' .` on a project root. It is easy to see that the --boot option is used in initdb for some historical reasons, but it is painful for a beginner like me. I believe the attached patch which adds a few comments might help a beginner.