doc: improve buf.indexOf() documentation style

* improve usage of "coerce" in buffer.md
* reduce informal tone in buffer.md
* wrap line at 80 characters in buffer.md
* wrap value in backtick for consistency

PR-URL: #19861
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

Author: Trott

doc/api/buffer.md

@@ -1259,10 +1259,10 @@ changes:
 
 * `value` {string|Buffer|Uint8Array|integer} What to search for.
 * `byteOffset` {integer} Where to begin searching in `buf`. **Default:** `0`.
-* `encoding` {string} If `value` is a string, this is its encoding.
+* `encoding` {string} The encoding of `value` if `value` is a string.
   **Default:** `'utf8'`.
-* Returns: {integer} The index of the first occurrence of `value` in `buf` or `-1`
-  if `buf` does not contain `value`.
+* Returns: {integer} The index of the first occurrence of `value` in `buf` or
+  `-1` if `buf` does not contain `value`.
 
 If `value` is:
 
@@ -1301,9 +1301,9 @@ If `value` is not a string, number, or `Buffer`, this method will throw a
 `TypeError`. If `value` is a number, it will be coerced to a valid byte value,
 an integer between 0 and 255.
 
-If `byteOffset` is not a number, it will be coerced to a number. Any arguments
-that coerce to `NaN` or 0, like `{}`, `[]`, `null` or `undefined`, will search
-the whole buffer. This behavior matches [`String#indexOf()`].
+If `byteOffset` is not a number, it will be coerced to a number. If the result
+of coercion is `NaN` or `0`, then the entire buffer will be searched. This
+behavior matches [`String#indexOf()`].
 
 ```js
 const b = Buffer.from('abcdef');