install: clarify fsync() used in safe (-S) mode

[marck]

Summary

install(1) uses fsync between writing to temporary file and renaming it to the target;
sometimes this is unclear as install -S could be slow in wdrain state while fsync()ing.

Clarify this moment in the man page a bit.

[mjg]

I think it would be nicer to add an option to do the rename bit but *not* fsync

poudriere is doing -S and it kills i/o perf