docs: document caveats of rev-list's object-name output

At first glance, the names given by "rev-list --objects" seem like a
good way to see which paths are present in a set of commits. But there
are some subtle gotchas there. We do not document the format of the
names at all, so let's do so, along with warning of these problems.

I intentionally did not document the exact format of the names here, as
I don't think it's something we want people to rely on (though I doubt
in practice that we'd change it at this point).

Though all of this is historically tied to "--objects", these days we
have a separate "--object-names" flag which can turn the names off or
on. So I put the detailed documentation there, but added a note from
--objects (which did not otherwise mention the names at all, even though
they are on by default).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/rev-list-options.txt

@@ -882,7 +882,7 @@ ifdef::git-rev-list[]
 	Print the object IDs of any object referenced by the listed
 	commits.  `--objects foo ^bar` thus means ``send me
 	all object IDs which I need to download if I have the commit
-	object _bar_ but not _foo_''.
+	object _bar_ but not _foo_''. See also `--object-names` below.
 
 --in-commit-order::
 	Print tree and blob ids in order of the commits. The tree
@@ -912,7 +912,12 @@ ifdef::git-rev-list[]
 
 --object-names::
 	Only useful with `--objects`; print the names of the object IDs
-	that are found. This is the default behavior.
+	that are found. This is the default behavior. Note that the
+	"name" of each object is ambiguous, and mostly intended as a
+	hint for packing objects. In particular: no distinction is made between
+	the names of tags, trees, and blobs; path names may be modified
+	to remove newlines; and if an object would appear multiple times
+	with different names, only one name is shown.
 
 --no-object-names::
 	Only useful with `--objects`; does not print the names of the object