Update docs

doc/api.md

@@ -549,14 +549,14 @@ fmt::print("{}", +s.bit);
 This is a known limitation of "perfect" forwarding in C++.
 
 <a id="compile-api"></a>
-## Format String Compilation
+## Compile-Time Support
 
 `fmt/compile.h` provides format string compilation and compile-time
 (`constexpr`) formatting enabled via the `FMT_COMPILE` macro or the `_cf`
 user-defined literal defined in namespace `fmt::literals`. Format strings
 marked with `FMT_COMPILE` or `_cf` are parsed, checked and converted into
 efficient formatting code at compile-time. This supports arguments of built-in
-and string types as well as user-defined types with `format` functions taking
+and string types as well as user-defined types with `format` methods taking
 the format context type as a template parameter in their `formatter`
 specializations. For example:
 
@@ -571,6 +571,29 @@ Format string compilation can generate more binary code compared to the
 default API and is only recommended in places where formatting is a
 performance bottleneck.
 
+The same API supports formatting at compile time e.g. in `constexpr` functions.
+It works with built-in and user-defined formatters that have `constexpr` `parse`
+and `format` methods. Example ([run](https://www.godbolt.org/z/rzY8Tcjf8)):
+
+    struct point {
+      double x;
+      double y;
+    };
+
+    template <> struct fmt::formatter<point> {
+      constexpr auto parse(format_parse_context& ctx) {
+        return ctx.begin();
+      }
+
+      template <typename FormatContext>
+      constexpr auto format(const point& p, FormatContext& ctx) const {
+        return format_to(ctx.out(), "({}, {})"_cf, p.x, p.y);
+      }
+    };
+
+    using namespace fmt::literals;
+    constexpr std::string s = fmt::format("{}"_cf, point(1, 2));
+
 ::: FMT_COMPILE
 
 ::: operator""_cf