Bug 1594449 - followup: fix some CSS loader comments. r=mayhemer

Relatedly: We only use this to determine priority. It seems we prioritize
<link rel=preload> over <link rel=stylesheet>, is that intended?

That seems a bit weird, as the preloads from the parser are likely to be used
very soon.

Differential Revision: https://phabricator.services.mozilla.com/D55225

--HG--
extra : moz-landing-system : lando

layout/style/Loader.h

@@ -205,12 +205,15 @@ class Loader final {
       nsIURI*, SheetParsingMode = eAuthorSheetFeatures,
       UseSystemPrincipal = UseSystemPrincipal::No);
 
-  /**
-   * Yes: this is a speculative load initiated by a <script> tag
-   * Link: this is a speculative load as well, but initiated by <link
-   *       rel="preload" as="style"> tag
-   */
-  enum class IsPreload { No, FromParser, FromLink };
+  enum class IsPreload {
+    No,
+    // This is a speculative load initiated by a <link rel=stylesheet> tag
+    // scanned by the parser, or @import rules found in a <style> tag.
+    FromParser,
+    // This is a speculative load as well, but initiated by
+    // <link rel="preload" as="style">
+    FromLink,
+  };
 
   /**
    * Asynchronously load the stylesheet at aURL.  If a successful result is