Add and correct information on P1383

Author: eisenwave

content/more-constexpr-cmath.md

@@ -0,0 +1,29 @@
+---
+execute: false
+---
+
+## What It Does
+
+Most functions in the `<cmath>` and `<complex>` headers are made `constexpr`.
+
+## Why It Matters
+
+Historically, while arithmetic expressions such as addition and multiplication
+between floating-point operands could be used in constant expressions,
+common mathematical functions such as `std::sqrt` or `std::pow` couldn't be.
+This meant that some computations had to be done unnecessarily at runtime,
+or the user had to re-implement a `constexpr` version
+of the `<cmath>`functions themselves.
+The same applies to 
+
+## Example
+
+```cpp
+#include <cmath>
+#include <complex>
+
+constexpr double sqrt_10 = std::sqrt(10);
+
+using namespace std::complex_literals;
+constexpr std::complex<double> i = std::sqrt(-1i);
+```

features_cpp26.yaml

@@ -309,8 +309,10 @@ features:
 
   - desc: "More constexpr for `<cmath>` and `<complex>`"
     paper: P1383
+    summary: "More mathematical functions are now `constexpr`, such as `std::sqrt`."
+    content: more-constexpr-cmath.md
     lib: true
-    support: [GCC 4.6]
+    support: [GCC 4.6 (partial)]
     ftm:
       - name: __cpp_lib_constexpr_cmath
         value: 202306L