Add comments for a postgres program in bootstrap mode

Hi,

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.

Many thanks,

--
Youki Shiraishi
NTT Software Innovation Center
Phone: +81-(0)3-5860-5115
Email: shiraishi(at)computer(dot)org