[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/react/components/web/Watermarks.js

@@ -8,7 +8,7 @@ import { translate } from '../../../i18n';
 declare var interfaceConfig: Object;
 
 /**
- * The CSS style of the element with CSS class <tt>rightwatermark</tt>.
+ * The CSS style of the element with CSS class {@code rightwatermark}.
  *
  * @private
  */