docs/git-blame: explain more clearly the example pickaxe use

We state that the following paragraph mentions the pickaxe
interface, but the term pickaxe is not then used. This
change clarifies that the example command uses the pickaxe
interface and what it is searching for.

Signed-off-by: Albert L. Lash, IV <alash3@bloomberg.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-blame.txt

@@ -35,7 +35,8 @@ Apart from supporting file annotation, Git also supports searching the
 development history for when a code snippet occurred in a change. This makes it
 possible to track when a code snippet was added to a file, moved or copied
 between files, and eventually deleted or replaced. It works by searching for
-a text string in the diff. A small example:
+a text string in the diff. A small example of the pickaxe interface
+that searches for `blame_usage`:
 
 -----------------------------------------------------------------------------
 $ git log --pretty=oneline -S'blame_usage'