docs: reword size param explanation

src/Discord.Net.Core/CDN.cs

@@ -23,7 +23,7 @@ namespace Discord
         /// </summary>
         /// <param name="userId">The user snowflake identifier.</param>
         /// <param name="avatarId">The avatar identifier.</param>
-        /// <param name="size">The size of the image to return in. This can be any power of two between 16 and 2048.</param>
+        /// <param name="size">The size of the image to return in horizontal pixels. This can be any power of two between 16 and 2048.</param>
         /// <param name="format">The format to return.</param>
         /// <returns>
         ///     A URL pointing to the user's avatar in the specified size.
@@ -82,7 +82,7 @@ namespace Discord
         /// </summary>
         /// <param name="guildId">The guild snowflake identifier.</param>
         /// <param name="vanityId">The vanity image identifier.</param>
-        /// <param name="size">The size of the image to return in. This can be any power of two between 16 and 2048.</param>
+        /// <param name="size">The size of the image to return in horizontal pixels. This can be any power of two between 16 and 2048.</param>
         /// <returns>
         ///     A URL pointing to the guild's vanity image.
         /// </returns>