Slightly improve initdb --sync-only option's help message

When reading the output of `initdb --help` I could not clearly
understand what the purpose of the --sync-only option was, until I
read the documentation of initdb.

-S, --sync-only only sync data directory

Perhaps the confusion was caused by the fact that sync(hronization)
means different things in different contexts, and many of those
contexts apply to databases, and to data directories; time sync, data
sync, replica sync, etc.

I think it would be helpful if the help message was slightly more
descriptive. Some options:

Used in patch:
only sync data directory; does not modify any data

To match the wording of --sync-only option:
write contents of data directory to disk; helpful after --no-sync option

Clearly specify the system operation used for the option
perform fsync on data directory; helpful after --no-sync option

Best regards,
--
Gurjeet Singh http://gurjeet.singh.im/