[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/base/util/loadScript.native.js

@@ -1,7 +1,7 @@
 /**
  * Loads a script from a specific URL. React Native cannot load a JS
  * file/resource/URL via a <script> HTML element, so the implementation
- * fetches the specified <tt>url</tt> as plain text using {@link fetch()} and
+ * fetches the specified {@code url} as plain text using {@link fetch()} and
  * then evaluates the fetched string as JavaScript code (using {@link eval()}).
  *
  * @param {string} url - The absolute URL from which the script is to be