docs: add git hash-object -t option's possible values

The summary under the NAME section for git hash-object can mislead
readers to conclude that the command can only be used to create blobs,
whereas the description makes it clear that it can be used to create
objects, not just blobs. Let's clarify the one-line summary.

Further, the description for the option -t does not list out other types
that can be used when creating objects. Let's make this explicit by
listing out the different object types.

Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-hash-object.txt

@@ -3,7 +3,7 @@ git-hash-object(1)
 
 NAME
 ----
-git-hash-object - Compute object ID and optionally creates a blob from a file
+git-hash-object - Compute object ID and optionally create an object from a file
 
 
 SYNOPSIS
@@ -25,7 +25,8 @@ OPTIONS
 -------
 
 -t <type>::
-	Specify the type (default: "blob").
+	Specify the type of object to be created (default: "blob"). Possible
+	values are `commit`, `tree`, `blob`, and `tag`.
 
 -w::
 	Actually write the object into the object database.