Document rule for attachment names in embeds (serenity-rs#2959)

Co-authored-by: Ivin <[email protected]>

src/builder/create_embed.rs

@@ -100,7 +100,10 @@ impl CreateEmbed {
         self
     }
 
-    /// Set the image associated with the embed. This only supports HTTP(S).
+    /// Set the image associated with the embed.
+    ///
+    /// Refer [Discord Documentation](https://discord.com/developers/docs/reference#uploading-files)
+    /// for rules on naming local attachments.
     #[inline]
     pub fn image(mut self, url: impl Into<String>) -> Self {
         self.0.image = Some(EmbedImage {
@@ -112,7 +115,7 @@ impl CreateEmbed {
         self
     }
 
-    /// Set the thumbnail of the embed. This only supports HTTP(S).
+    /// Set the thumbnail of the embed.
     #[inline]
     pub fn thumbnail(mut self, url: impl Into<String>) -> Self {
         self.0.thumbnail = Some(EmbedThumbnail {
@@ -163,6 +166,8 @@ impl CreateEmbed {
     /// Note however, you have to be sure you set an attachment (with [`ChannelId::send_files`])
     /// with the provided filename. Or else this won't work.
     ///
+    /// Refer [`Self::image`] for rules on naming local attachments.
+    ///
     /// [`ChannelId::send_files`]: crate::model::id::ChannelId::send_files
     #[inline]
     pub fn attachment(self, filename: impl Into<String>) -> Self {
@@ -299,7 +304,9 @@ impl CreateEmbedFooter {
         self
     }
 
-    /// Set the icon URL's value. This only supports HTTP(S).
+    /// Set the icon URL's value.
+    ///
+    /// Refer [`CreateEmbed::image`] for rules on naming local attachments.
     pub fn icon_url(mut self, icon_url: impl Into<String>) -> Self {
         self.0.icon_url = Some(icon_url.into());
         self