Update -L documentation for git-blame/git-annotate

Documenting alternate ways to use -L:

-L /regex/,end
-L start,+offset

Signed-off-by: Andrew Ruder <andy@aeruder.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/blame-options.txt

@@ -9,8 +9,28 @@
 --show-stats::
 	Include additional statistics at the end of blame output.
 
--L n,m::
-	Annotate only the specified line range (lines count from 1).
+-L <start>,<end>::
+	Annotate only the given line range.  <start> and <end> can take
+	one of these forms:
+
+	- number
++
+If <start> or <end> is a number, it specifies an
+absolute line number (lines count from 1).
++
+
+- /regex/
++
+This form will use the first line matching the given
+POSIX regex.  If <end> is a regex, it will search
+starting at the line given by <start>.
++
+
+- +offset or -offset
++
+This is only valid for <end> and will specify a number
+of lines before or after the line given by <start>.
++
 
 -l::
 	Show long rev (Default: off).