doc: cleanup pendingSettingsAck docs

This commit rephrases the first sentence of the http2 session
pendingSettingsAck property docs. It also formats SETTINGS
consistently.

PR-URL: #28388
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/http2.md

@@ -451,9 +451,9 @@ added: v8.4.0
 
 * {boolean}
 
-Indicates whether or not the `Http2Session` is currently waiting for an
-acknowledgment for a sent `SETTINGS` frame. Will be `true` after calling the
-`http2session.settings()` method. Will be `false` once all sent SETTINGS
+Indicates whether the `Http2Session` is currently waiting for acknowledgment of
+a sent `SETTINGS` frame. Will be `true` after calling the
+`http2session.settings()` method. Will be `false` once all sent `SETTINGS`
 frames have been acknowledged.
 
 #### http2session.ping([payload, ]callback)