Document differences between hardened std::span and gsl::span (#1206)

* Initial plan for issue

* Document differences between hardened std::span and gsl::span

Co-authored-by: carsonRadtke <10507970+carsonRadtke@users.noreply.github.com>

* Simplify documentation for span differences

Co-authored-by: carsonRadtke <10507970+carsonRadtke@users.noreply.github.com>

* Enhance span documentation with comparison table and links

Co-authored-by: carsonRadtke <10507970+carsonRadtke@users.noreply.github.com>

* Fix library hardening documentation links

Co-authored-by: carsonRadtke <10507970+carsonRadtke@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: carsonRadtke <10507970+carsonRadtke@users.noreply.github.com>

docs/headers.md

@@ -412,22 +412,36 @@ The `gsl::span` is based on the standardized version of `std::span` which was ad
 deprecate `gsl::span` when `std::span` finished standardization, however that plan changed when the runtime bounds checking
 was removed from `std::span`'s design.
 
-The only difference between `gsl::span` and `std::span` is that `gsl::span` strictly enforces runtime bounds checking.
-Any violations of the bounds check results in termination of the program.
-Like `gsl::span`, `gsl::span`'s iterators also differ from `std::span`'s iterator in that all access operations are bounds checked.
+The key differences between `gsl::span` and `std::span` are:
+- `gsl::span` strictly enforces runtime bounds checking for all access operations
+- Any violations of the bounds check results in termination of the program
+- `gsl::span`'s iterators also perform bounds checking, unlike `std::span`'s iterators
 
 #### Which version of span should I use?
 
-##### Use `gsl::span` if
+The following table compares the different span implementations to help you choose which one is best for your project:
 
-- you want to guarantee bounds safety in your project.
-  - All data accessing operations use bounds checking to ensure you are only accessing valid memory.
-- your project uses C++14 or C++17.
-  - `std::span` is not available as it was not introduced into the STL until C++20.
+| Feature/Version | `std::span` (C++20/23) | Hardened `std::span` (C++26) | `gsl::span` |
+|-----------------|------------------------|------------------------------|-------------|
+| **C++ Standard** | Requires C++20 or later | Requires C++26 or backported implementation | Works with C++14 or later |
+| **Element Access** | No bounds checking | Bounds checking | Bounds checking |
+| **Iterator Safety** | No bounds checking | Implementation-defined, may depend on vendor | Full bounds checking |
+| **Error Behavior** | Undefined behavior on invalid access | Implementation-defined, may be configurable | Always calls [`std::terminate()`](https://en.cppreference.com/w/cpp/error/terminate) via [gsl::details::terminate()](https://github.com/microsoft/GSL/blob/main/include/gsl/assert#L111-L118) |
+| **Performance** | Fastest (no checking) | Varies by implementation and configuration | May have performance impact from bounds checking |
 
-##### Use `std::span` if
+**Recommendations:**
 
-- your project is C++20 and you need the performance offered by `std::span`.
+- **C++14 & C++17 projects**: Use `gsl::span` as `std::span` is not available.
+- **C++20 & C++23 projects**: 
+  - Use `gsl::span` if safety is your priority.
+  - Use `std::span` if performance is critical and you're confident in your index calculations.
+- **C++26 projects**: 
+  - Use `gsl::span` if you need guaranteed iterator safety across all platforms.
+  - Use hardened `std::span` if you want standard library compliance and acceptable safety.
+
+**Implementation notes for hardened `std::span` in C++26:**
+- For MSVC: See [Microsoft STL Hardening](https://github.com/microsoft/STL/wiki/STL-Hardening)
+- For Clang/LLVM: See [libc++ Hardening](https://libcxx.llvm.org/Hardening.html)
 
 #### Types