Improve RequestBuilder.form() docs (#1490)

src/async_impl/request.rs

@@ -375,6 +375,32 @@ impl RequestBuilder {
     }
 
     /// Send a form body.
+    ///
+    /// Sets the body to the url encoded serialization of the passed value,
+    /// and also sets the `Content-Type: application/x-www-form-urlencoded`
+    /// header.
+    ///
+    /// ```rust
+    /// # use reqwest::Error;
+    /// # use std::collections::HashMap;
+    /// #
+    /// # async fn run() -> Result<(), Error> {
+    /// let mut params = HashMap::new();
+    /// params.insert("lang", "rust");
+    ///
+    /// let client = reqwest::Client::new();
+    /// let res = client.post("http://httpbin.org")
+    ///     .form(&params)
+    ///     .send()
+    ///     .await?;
+    /// # Ok(())
+    /// # }
+    /// ```
+    ///
+    /// # Errors
+    ///
+    /// This method fails if the passed value cannot be serialized into
+    /// url encoded format
     pub fn form<T: Serialize + ?Sized>(mut self, form: &T) -> RequestBuilder {
         let mut error = None;
         if let Ok(ref mut req) = self.request {