Obsolete reference to TransactionError::UserReturnedError in docs

[killercup]

See:

diesel/diesel/src/connection/mod.rs

Lines 33 to 36 in 6b8d35c

	/// If the function returns an `Ok`, that value will be returned. If the
	/// function returns an `Err`,
	/// [`TransactionError::UserReturnedError`](../result/enum.TransactionError.html#variant.UserReturnedError)
	/// will be returned wrapping that value.

Should probably also add an example when updating this.

[Eijebong]

6a932c3

This is not true anymore. We still need an example though