book: cover remaining minor C++23 language improvements

Add a 11.1 subsection on smaller C++23 language features: #elifdef/
#elifndef/#warning, named universal character escapes, extended
floating-point types (<stdfloat>), and further constexpr relaxations.
Refs #317

Author: changkun

book/en-us/11-cpp23.md

@@ -86,6 +86,15 @@ int divide_by(int x) {
 }
 ```
 
+### Other small language improvements
+
+C++23 also includes several smaller language improvements:
+
+- **Preprocessor directives**: added `#elifdef` / `#elifndef` (shorthands for `#elif defined` / `#elif !defined`) and `#warning` for emitting a diagnostic on purpose.
+- **Named universal character escapes**: a character can be written by its Unicode name, e.g. `"\N{GREEK SMALL LETTER ALPHA}"`, which is more readable than memorizing code points.
+- **Extended floating-point types**: `<stdfloat>` introduces fixed-width floating-point types such as `std::float16_t`, `std::float32_t`, `std::float64_t`, and `std::bfloat16_t` (their availability likewise depends on the platform and standard-library implementation).
+- **Further `constexpr` relaxations**: constant evaluation now permits `goto` and labeled statements, as well as declaring (but not using during constant evaluation) variables of non-literal type and `static` variables.
+
 ## 11.2 The standard library
 
 ### `std::expected`

book/zh-cn/11-cpp23.md

@@ -86,6 +86,15 @@ int divide_by(int x) {
 }
 ```
 
+### 其他语言层面的小改进
+
+C++23 还包含若干较小的语言改进：
+
+- **预处理指令**：新增了 `#elifdef` / `#elifndef`（即 `#elif defined` / `#elif !defined` 的简写）以及用于主动产生诊断信息的 `#warning`。
+- **命名的通用字符转义**：可以用字符的 Unicode 名称来书写字符，例如 `"\N{GREEK SMALL LETTER ALPHA}"`，比记忆码位更直观。
+- **扩展浮点类型**：在 `<stdfloat>` 中引入了 `std::float16_t`、`std::float32_t`、`std::float64_t`、`std::bfloat16_t` 等定宽浮点类型（其可用性同样取决于平台与标准库实现）。
+- **`constexpr` 的进一步放宽**：允许在常量求值中使用 `goto`、带标签的语句，以及声明（但不在常量求值期间使用）非字面类型的变量与 `static` 变量等。
+
 ## 11.2 标准库
 
 ### `std::expected`