Fix some broken rustdoc links in core::panic documentation

Author: dtolnay

library/core/src/panic/location.rs

@@ -4,6 +4,8 @@ use crate::fmt;
 ///
 /// This structure is created by [`PanicInfo::location()`].
 ///
+/// [`PanicInfo::location()`]: crate::panic::PanicInfo::location
+///
 /// # Examples
 ///
 /// ```should_panic

library/core/src/panic/unwind_safe.rs

@@ -15,6 +15,8 @@ use crate::task::{Context, Poll};
 /// purpose of this trait is to encode what types are safe to cross a [`catch_unwind`]
 /// boundary with no fear of unwind safety.
 ///
+/// [`catch_unwind`]: ../../std/panic/fn.catch_unwind.html
+///
 /// ## What is unwind safety?
 ///
 /// In Rust a function can "return" early if it either panics or calls a
@@ -116,6 +118,8 @@ pub auto trait RefUnwindSafe {}
 /// account. This wrapper struct is useful for a quick and lightweight
 /// annotation that a variable is indeed unwind safe.
 ///
+/// [`catch_unwind`]: ../../std/panic/fn.catch_unwind.html
+///
 /// # Examples
 ///
 /// One way to use `AssertUnwindSafe` is to assert that the entire closure