blame-options.txt: explain that -L <start> and <end> are optional

The ability to omit either end of the -L range is a handy but
undocumented shortcut, and is thus not easily discovered. Fix this
shortcoming.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/blame-options.txt

@@ -11,8 +11,11 @@
 
 -L <start>,<end>::
 -L :<regex>::
-	Annotate only the given line range.  <start> and <end> can take
-	one of these forms:
+	Annotate only the given line range.  <start> and <end> are optional.
+	``-L <start>'' or ``-L <start>,'' spans from <start> to end of file.
+	``-L ,<end>'' spans from start of file to <end>.
++
+<start> and <end> can take one of these forms:
 
 include::line-range-format.txt[]