Added error documentation for write_fmt

This continuation of work at rust-lang#98861

library/core/src/fmt/mod.rs

@@ -174,6 +174,11 @@ fn write_char(&mut self, c: char) -> Result {
     /// This method should generally not be invoked manually, but rather through
     /// the [`write!`] macro itself.
     ///
+    /// # Errors
+    ///
+    /// This function will return an instance of [`Error`] on error. Please see
+    /// [write_str](Write::write_str) for details.
+    ///
     /// # Examples
     ///
     /// ```