doc: update path.extname documentation

per: nodejs/node-v0.x-archive#8509 originally submitted by @thauburger Adding an additional example to path.extname documentation to demonstrate the case where the first character of the last path component is '.'. This case is interesting, as something like path.extname('.txt') returns an empty string. In this case, .txt can be used as a valid file name (while arguably maintaining an extension). I agree with Node's behavior in this case, but I think the added example provides additional clarity for the developer.
nodejs · jasnell · Aug 14, 2015 · Aug 14, 2015 · Aug 14, 2015 · Aug 14, 2015
commit 3d5056faa7c98e1d82e9744f2769b98240ac0a03
diff --git a/doc/api/path.markdown b/doc/api/path.markdown
@@ -183,6 +183,10 @@ an empty string.  Examples:
     // returns
     ''
 
+    path.extname('.index')
+    // returns
+    ''
+
 ## path.sep
 
 The platform-specific file separator. `'\\'` or `'/'`.