core.fsyncObjectFiles::
+	This boolean will enable 'fsync()' when writing loose object
+	files. Both the file itself and its containng directory will
+	be fsynced.
++
+When git writes data any required object writes will precede the
+corresponding reference update(s). For example, a
+linkgit:git-receive-pack[1] accepting a push might write a pack or
+loose objects (depending on settings such as `transfer.unpackLimit`).
++
+Therefore on a journaled file system which ensures that data is
+flushed to disk in chronological order an fsync shouldn't be
+needed. The loose objects might be lost with a crash, but so will the
+ref update that would have referenced them. Git's own state in such a
+crash will remain consistent.

While this is much better than what we had before I'm not sure it is
all that useful.  The only file system I know of that actually had the
above behavior was ext3, and the fact that it always wrote back that
way made it a complete performance desaster.  So even mentioning this
here will probably create a lot more confusion than actually clearing
things up.

++
+This option exists because that assumption doesn't hold on filesystems
+where the data ordering is not preserved, such as on ext3 and ext4
+with "data=writeback". On such a filesystem the `rename()` that drops
+the new reference in place might be preserved, but the contents or
+directory entry for the loose object(s) might not have been synced to
+disk.

As well as just about any other file system.  Which is another argument
on why it needs to be on by default.  Every time I install a new
development system (aka one that often crashes) and forget to enable
the option I keep corrupting my git repos.  And that is with at least
btrfs, ext4 and xfs as it is pretty much by design.

+However, that's highly filesystem-dependent, on some filesystems
+simply calling fsync() might force an unrelated bulk background write
+to be serialized to disk. Such edge cases are the reason this option
+is off by default. That default setting might change in future
+versions.

Again the only "some file system" that was widely used that did this
was ext3.  And ext3 has long been removed from the Linux kernel..