Add Missing Functionality in callback.md

Change-Id: I662bce08782a154da3971f6046b40570c103fa35
Reviewed-on: https://chromium-review.googlesource.com/886825
Commit-Queue: Gayane Petrosyan <gayane@chromium.org>
Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542115}

docs/callback.md

@@ -545,6 +545,12 @@ void Foo(const char* ptr);
 void Bar(char* ptr);
 base::Bind(&Foo, "test");
 base::Bind(&Bar, "test");  // This fails because ptr is not const.
+```
+ - In case of partial binding of parameters a possibility of having unbound
+   parameters before bound parameters. Example:
+```cpp
+void Foo(int x, bool y);
+base::Bind(&Foo, _1, false); // _1 is a placeholder.
 ```
 
 If you are thinking of forward declaring `base::Callback` in your own header