uart.4: describe better + spdx

+ add `serial` to description (search keywords)
+ remove redundant `(UART)`, `for`, and `device`
+ description is now one line on standard cons.

MFC after:	3 days
Reviewed by:	imp
Pull Request:	https://github.com/freebsd/freebsd-src/pull/1531

(cherry picked from commit db512bb303)

share/man/man4/uart.4

@@ -1,3 +1,5 @@
+.\"-
+.\" SPDX-License-Identifier: BSD-2-Clause
 .\"
 .\" Copyright (c) 2003 Marcel Moolenaar
 .\" All rights reserved.
@@ -23,12 +25,12 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd July 11, 2020
+.Dd December 5, 2024
 .Dt UART 4
 .Os
 .Sh NAME
 .Nm uart
-.Nd driver for Universal Asynchronous Receiver/Transmitter (UART) devices
+.Nd Universal Asynchronous Receiver/Transmitter serial driver
 .Sh SYNOPSIS
 .Cd "device uart"
 .Pp