config.txt: clarify that add.ignore-errors is deprecated

The old text gave an impression that even in a new repository using
old form might be safer.  Only Git from pre 1.7.0 days choke on the
correctly named variable, which is ancient by today's standard.

We have no intention to remove the support for deprecated ones, but
let's make sure that we do not give room for confused questions such
as "why does core.sparse-checkout not work, when add.ignore-errors
does?"

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -664,14 +664,13 @@ core.abbrev::
 	for abbreviated object names to stay unique for sufficiently long
 	time.
 
-add.ignore-errors::
 add.ignoreErrors::
+add.ignore-errors::
 	Tells 'git add' to continue adding files when some files cannot be
 	added due to indexing errors. Equivalent to the '--ignore-errors'
-	option of linkgit:git-add[1].  Older versions of Git accept only
-	`add.ignore-errors`, which does not follow the usual naming
-	convention for configuration variables.  Newer versions of Git
-	honor `add.ignoreErrors` as well.
+	option of linkgit:git-add[1].  `add.ignore-errors` is deprecated,
+	as it does not follow the usual naming convention for configuration
+	variables.
 
 alias.*::
 	Command aliases for the linkgit:git[1] command wrapper - e.g.