Clarify initdb --sync-only help message and docs

Clarify initdb --sync-only help message and docs

The initdb help message for --sync-only was a bit terse, and not
really self-explanatory. Make it clearer that initdb --sync-only
will exit after syncing, and expand the docs with a note on when
the option can be useful. Also align the help output with others
that exit immediately.

Author: Nathan Bossart, Gurjeet Singh
Discussion: https://postgr.es/m/CABwTF4U6hbNNE1bv=LxQdJybmUdZ5NJQ9rKY9tN82NXM8QH+iQ@mail.gmail.com