config: document blame configuration

The options are currently only referenced by the git-blame man page,
also explain them in git-config, which is the canonical page to
contain all config options.
Signed-off-by: NStefan Beller <sbeller@google.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -943,6 +943,23 @@ apply.whitespace::
 	Tells 'git apply' how to handle whitespaces, in the same way
 	as the `--whitespace` option. See linkgit:git-apply[1].
 
+blame.showRoot::
+	Do not treat root commits as boundaries in linkgit:git-blame[1].
+	This option defaults to false.
+
+blame.blankBoundary::
+	Show blank commit object name for boundary commits in
+	linkgit:git-blame[1]. This option defaults to false.
+
+blame.showEmail::
+	Show the author email instead of author name in linkgit:git-blame[1].
+	This option defaults to false.
+
+blame.date::
+	Specifies the format used to output dates in linkgit:git-blame[1].
+	If unset the iso format is used. For supported values,
+	see the discussion of the `--date` option at linkgit:git-log[1].
+
 branch.autoSetupMerge::
 	Tells 'git branch' and 'git checkout' to set up new branches
 	so that linkgit:git-pull[1] will appropriately merge from the