doc: update readline asyncIterator docs

This commit:
- Removes an unnecessary stability index entry. These generally
  are not included for Stable entries.
- Remove mention of experimental status that is not true anymore.
- Remove use of "we"
- Remove use of relative time phrasing.
- Misc cleanup.

PR-URL: #28425
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: cjihrig

doc/api/readline.md

@@ -318,8 +318,6 @@ changes:
     description: Symbol.asyncIterator support is no longer experimental.
 -->
 
-> Stability: 2 - Stable
-
 * Returns: {AsyncIterator}
 
 Create an `AsyncIterator` object that iterates through each line in the input
@@ -332,10 +330,8 @@ If the loop is terminated with `break`, `throw`, or `return`,
 [`rl.close()`][] will be called. In other words, iterating over a
 `readline.Interface` will always consume the input stream fully.
 
-A caveat with using this experimental API is that the performance is
-currently not on par with the traditional `'line'` event API, and thus it is
-not recommended for performance-sensitive applications. We expect this
-situation to improve in the future.
+Performance is not on par with the traditional `'line'` event API. Use `'line'`
+instead for performance-sensitive applications.
 
 ```js
 async function processLineByLine() {