doc: improve buf.lastIndexOf() text

Improve description of `encoding` option. (This change is also applied
to `buf.indexOf()`.) Improve punctuation. Wrap at 80 characters.

PR-URL: #19904
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

doc/api/buffer.md

@@ -1240,9 +1240,10 @@ changes:
 
 * `value` {string|Buffer|Uint8Array|integer} What to search for.
 * `byteOffset` {integer} Where to begin searching in `buf`. **Default:** `0`.
-* `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
+* `encoding` {string} If `value` is a string, this is the encoding used to
+  determine the binary representation of the string that will be searched for in
+  `buf`. **Default:** `'utf8'`.
+* Returns: {integer} The index of the first occurrence of `value` in `buf`, or
   `-1` if `buf` does not contain `value`.
 
 If `value` is:
@@ -1342,13 +1343,14 @@ changes:
 * `value` {string|Buffer|Uint8Array|integer} What to search for.
 * `byteOffset` {integer} Where to begin searching in `buf`.
   **Default:** [`buf.length`]` - 1`.
-* `encoding` {string} If `value` is a string, this is its encoding.
-  **Default:** `'utf8'`.
-* Returns: {integer} The index of the last occurrence of `value` in `buf` or `-1`
-  if `buf` does not contain `value`.
+* `encoding` {string} If `value` is a string, this is the encoding used to
+  determine the binary representation of the string that will be searched for in
+  `buf`. **Default:** `'utf8'`.
+* Returns: {integer} The index of the last occurrence of `value` in `buf`, or
+  `-1` if `buf` does not contain `value`.
 
-Identical to [`buf.indexOf()`], except `buf` is searched from back to front
-instead of front to back.
+Identical to [`buf.indexOf()`], except the last occurrence of `value` is found
+rather than the first occurrence.
 
 ```js
 const buf = Buffer.from('this buffer is a buffer');