fast-import: Document author/committer/tagger name is optional

The fast-import parser does not validate that the author, committer
or tagger name component contains both a name and an email address.
Therefore the name component has always been optional.  Correct the
documentation to match the implementation.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/git-fast-import.txt b/Documentation/git-fast-import.txt
index 288032c..e6d364f 100644
--- a/Documentation/git-fast-import.txt
+++ b/Documentation/git-fast-import.txt
@@ -311,8 +311,8 @@
 ....
 	'commit' SP <ref> LF
 	mark?
-	('author' SP <name> SP LT <email> GT SP <when> LF)?
-	'committer' SP <name> SP LT <email> GT SP <when> LF
+	('author' (SP <name>)? SP LT <email> GT SP <when> LF)?
+	'committer' (SP <name>)? SP LT <email> GT SP <when> LF
 	data
 	('from' SP <committish> LF)?
 	('merge' SP <committish> LF)?
@@ -657,7 +657,7 @@
 ....
 	'tag' SP <name> LF
 	'from' SP <committish> LF
-	'tagger' SP <name> SP LT <email> GT SP <when> LF
+	'tagger' (SP <name>)? SP LT <email> GT SP <when> LF
 	data
 ....