Fix typos & add documentation (#3569)

* fix typo

* tips & typos

* improve doc string

* update documentation

* fmt

* fix typo

src/mimefactory.rs

@@ -445,6 +445,8 @@ impl<'a> MimeFactory<'a> {
             .collect()
     }
 
+    /// Consumes a `MimeFactory` and renders it into a message which is then stored in
+    /// `smtp`-table to be used by the SMTP loop
     pub async fn render(mut self, context: &Context) -> Result<RenderedEmail> {
         let mut headers: MessageHeaders = Default::default();