cu.1: describe better + tag spdx

[concussious]

Proposal: 0: cu(1) - call UNIX serial console

Alternative ideas: 1: cu(1) - call UNIX opens a serial console 2: cu(1) - call UNIX over a serial console

Goal: Preserve over 40 years of "call UNIX" BSD heritage while answering "one line about what it does" in 2024:

"how do I get a serial console?" "% apropos serial"

[bsdimp]

"call unix over a serial line" maybe?

[concussious]

If you have time to merge this within the window, may we please have this in 14.2?

Of course this is quite minor of a feature, but being able to find our serial client with apropos serial is a feature which I hope will:

1. For users asking for recommended serial client, I can teach them to search the manual instead.
2. Increase user engagement with the manual subsystem, promoting user awareness and appreciation of BSD design philosophy.