Add JSDoc comments to various utility functions and core files.

This change adds JSDoc-style comments to several functions and classes across the codebase to improve readability and maintainability.

Comments were added to:
- `src/SEQTA.ts`: Explained the `init()` function.
- `src/seqta/utils/waitForElm.ts`: Detailed the `waitForElm()` function, its parameters, and behavior.
- `src/seqta/utils/stringToHTML.ts`: Clarified the `stringToHTML()` function, including its sanitization and styling features.
- `src/seqta/utils/delay.ts`: Added a brief explanation for the `delay()` utility.
- `src/seqta/utils/mutex.ts`: Documented the `Mutex` class and its `acquire` method (renamed from `lock`), explaining its asynchronous locking mechanism and the role of the returned unlock function.

src/seqta/utils/stringToHTML.ts

@@ -1,5 +1,17 @@
 import DOMPurify from "dompurify";
 
+/**
+ * Converts an HTML string into a DOM element, with sanitization and optional styling.
+ *
+ * This function first sanitizes the input HTML string using DOMPurify to prevent XSS attacks.
+ * The sanitization process allows 'onclick' attributes and specific URI schemes.
+ * Then, it parses the sanitized string into an HTML document and returns its body.
+ * Optionally, it can apply predefined CSS styles to the body element.
+ *
+ * @param {string} str The HTML string to convert.
+ * @param {boolean} [styles=false] Whether to apply predefined styles to the document body.
+ * @returns {HTMLElement} The body element of the parsed and sanitized HTML document.
+ */
 export default function stringToHTML(str: string, styles = false) {
   const parser = new DOMParser();