Case explaination is quite out of date.
I studied what openbsd and netbsd are doing, and try to fix it.
- show the optional left parenthesis and optional list
- explain there can be a space or not after the pattern
- use markup
- split the paragraphs
- remove totally unnecessary actually (akshually)
- import the exit status line from netbsd
While here, remove the '-' from the beginning of this document.
PR: 298329
Fixes: c9afaa63894e ("Add case statement fallthrough")
Fixes: f7a9b7fe3a8d ("Allow a left parenthesis before patterns in case blocks")
Fixes: e00e16ad7f86 ("Allow empty case/esac statements")
MFC after: 3 days