[RN] Fix documentation comments

* Javadoc introduced @code as a replacement of <code> and <tt> which is
  better aligned with other javadoc tags such as @link. Use it in the
  Java source code. If we switch to Kotlin, then we'll definitely use
  Markdown.

* There are more uses of @code in the JavaScript source code than <tt>
  so use @code for the sake of consistency. Eventually, I'd rather we
  switch to Markdown because it's easier on my eyes.

* Xcode is plain confused by @code and @link. The Internet says that
  Xcode supports the backquote character to denote the beginning and end
  of a string of characters which should be formatted for display as
  code but it doesn't work for me. <tt> is not rendered at all. So use
  the backquote which is rendered itself. Hopefully, if we switch to
  Markdown, then it'll be common between JavaScript and Objective-C
  source code.

react/features/mobile/external-api/middleware.js

@@ -58,11 +58,11 @@ MiddlewareRegistry.register(store => next => action => {
 });
 
 /**
- * Gets the description of a specific <tt>Symbol</tt>.
+ * Gets the description of a specific {@code Symbol}.
  *
- * @param {Symbol} symbol - The <tt>Symbol</tt> to retrieve the description of.
+ * @param {Symbol} symbol - The {@code Symbol} to retrieve the description of.
  * @private
- * @returns {string} The description of <tt>symbol</tt>.
+ * @returns {string} The description of {@code symbol}.
  */
 function _getSymbolDescription(symbol: Symbol) {
     let description = symbol.toString();