doc: reword poolSize explanation in buffer.md

The original explanation was overly wordy, contained mildly questionable
punctuation, and did not wrap at 80 characters. This rewording fixes all
of these issues.

PR-URL: #19785
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

Author: Trott

doc/api/buffer.md

@@ -918,8 +918,8 @@ added: v0.11.3
 
 * {integer} **Default:** `8192`
 
-This is the number of bytes used to determine the size of pre-allocated, internal
-`Buffer` instances used for pooling. This value may be modified.
+This is the size (in bytes) of pre-allocated internal `Buffer` instances used
+for pooling. This value may be modified.
 
 ### buf[index]
 <!-- YAML