pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ea499f3d28c657a044f0a948e6b77ac56f28a8f6

Modified Files
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doc/src/sgml/ref/initdb.sgml | 3 +++
src/bin/initdb/initdb.c      | 2 +-
2 files changed, 4 insertions(+), 1 deletion(-)