Mention in the docs, that assert! has a second version with a custom message

I recently discovered that this is not mentioned in the docs, only in
the examples, and it's not evident for people coming from C++

r? @steveklabnik

src/libcore/macros.rs

@@ -34,6 +34,8 @@ macro_rules! panic {
 /// This will invoke the `panic!` macro if the provided expression cannot be
 /// evaluated to `true` at runtime.
 ///
+/// This macro has a second version, where a custom panic message can be provided.
+///
 /// # Examples
 ///
 /// ```
@@ -99,6 +101,9 @@ macro_rules! assert_eq {
 /// This will invoke the `panic!` macro if the provided expression cannot be
 /// evaluated to `true` at runtime.
 ///
+/// Like `assert!`, this macro also has a second version, where a custom panic
+/// message can be provided.
+///
 /// Unlike `assert!`, `debug_assert!` statements are only enabled in non
 /// optimized builds by default. An optimized build will omit all
 /// `debug_assert!` statements unless `-C debug-assertions` is passed to the