Add manpage links for FreeBSD headers mentioned in documentation. (#704)

Follow-up to #685. Add links to FreeBSD manpages on freebsd.org for the
header files we mention in our exit test documentation.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Sources/Testing/ExitTests/ExitCondition.swift

@@ -42,7 +42,7 @@ public enum ExitCondition: Sendable {
   /// |-|-|
   /// | macOS | [`<stdlib.h>`](https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/_Exit.3.html), [`<sysexits.h>`](https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/sysexits.3.html) |
   /// | Linux | [`<stdlib.h>`](https://sourceware.org/glibc/manual/latest/html_node/Exit-Status.html), `<sysexits.h>` |
-  /// | FreeBSD | `<stdlib.h>`, `<sysexits.h>` |
+  /// | FreeBSD | [`<stdlib.h>`](https://man.freebsd.org/cgi/man.cgi?exit(3)), [`<sysexits.h>`](https://man.freebsd.org/cgi/man.cgi?sysexits(3)) |
   /// | Windows | [`<stdlib.h>`](https://learn.microsoft.com/en-us/cpp/c-runtime-library/exit-success-exit-failure) |
   ///
   /// On macOS, FreeBSD, and Windows, the full exit code reported by the process
@@ -63,7 +63,7 @@ public enum ExitCondition: Sendable {
   /// |-|-|
   /// | macOS | [`<signal.h>`](https://developer.apple.com/library/archive/documentation/System/Conceptual/ManPages_iPhoneOS/man3/signal.3.html) |
   /// | Linux | [`<signal.h>`](https://sourceware.org/glibc/manual/latest/html_node/Standard-Signals.html) |
-  /// | FreeBSD | `<signal.h>` |
+  /// | FreeBSD | [`<signal.h>`](https://man.freebsd.org/cgi/man.cgi?signal(3)) |
   /// | Windows | [`<signal.h>`](https://learn.microsoft.com/en-us/cpp/c-runtime-library/signal-constants) |
   ///
   /// On Windows, by default, the C runtime will terminate a process with exit