docs(fmt): clarify runtime compatiblity (#6648)

Author: kt3k

fmt/README.md

@@ -11,3 +11,12 @@ import { red } from "@std/fmt/colors";
 
 console.log(red(format(1337))); // Prints "1.34 kB"
 ```
+
+# Runtime compatibility
+
+[bytes](https://jsr.io/@std/fmt/doc/bytes/~),
+[colors](https://jsr.io/@std/fmt/doc/colors/~), and
+[duration](https://jsr.io/@std/fmt/doc/duration/~) supports all major runtimes.
+[printf](https://jsr.io/@std/fmt/doc/printf/~) is mostly compatible with major
+runtimes, however some of features, such as `%v`, `%i` and `%I` format
+specifiers, are only available in Deno. See the API docs for details.

fmt/printf.ts

@@ -954,6 +954,9 @@ class Printf {
  *
  * See the module documentation for the available format strings.
  *
+ * `%v`, `%i`, and `%` are only supported in Deno runtime. Other formats are supported in
+ * other major runtimes.
+ *
  * @example Usage
  * ```ts
  * import { sprintf } from "@std/fmt/printf";
@@ -983,6 +986,8 @@ export function sprintf(format: string, ...args: unknown[]): string {
  *
  * See the module documentation for the available format strings.
  *
+ * This API only supports Deno runtime.
+ *
  * @example Usage
  * ```ts no-assert
  * import { printf } from "@std/fmt/printf";