Explain difference between panic! and abort in abort docs

As per rust-lang#29370

Author: Pirh

src/libstd/process.rs

@@ -1124,7 +1124,14 @@ pub fn exit(code: i32) -> ! {
 ///
 /// Note that because this function never returns, and that it terminates the
 /// process, no destructors on the current stack or any other thread's stack
-/// will be run. If a clean shutdown is needed it is recommended to only call
+/// will be run.
+///
+/// This is in contrast to the default behaviour of [`panic!`] which unwinds
+/// the current thread's stack and calls all destructors.
+/// When `panic="abort"` is set, either as an argument to `rustc` or in a
+/// crate's Cargo.toml, [`panic!`] and `abort` are equivalent.
+///
+/// If a clean shutdown is needed it is recommended to only call
 /// this function at a known point where there are no more destructors left
 /// to run.
 ///
@@ -1162,6 +1169,8 @@ pub fn exit(code: i32) -> ! {
 ///     // the destructor implemented for HasDrop will never get run
 /// }
 /// ```
+///
+/// [`panic!`]: ../../std/macro.panic.html
 #[stable(feature = "process_abort", since = "1.17.0")]
 pub fn abort() -> ! {
     unsafe { ::sys::abort_internal() };