Improve RequestBuilder.form() docs

[nihaals]

Based on:

reqwest/src/blocking/request.rs

Lines 418 to 443 in 0170947

	/// 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;
	/// #
	/// # fn run() -> Result<(), Error> {
	/// let mut params = HashMap::new();
	/// params.insert("lang", "rust");
	///
	/// let client = reqwest::blocking::Client::new();
	/// let res = client.post("http://httpbin.org")
	/// .form(&params)
	/// .send()?;
	/// # Ok(())
	/// # }
	/// ```
	///
	/// # Errors
	///
	/// This method fails if the passed value cannot be serialized into
	/// url encoded format

[seanmonstar]

Thanks so much!