checkpatch: describe --min-conf-desc-length

checkpatch: describe --min-conf-desc-length

Neither the warning nor the help message gives any hint on the unit for
length: Could be meters, inches, bytes, characters or ...  lines.

Extend the output of `--help` to name the unit "lines" and the default:
-  --min-conf-desc-length=n   set the min description length, if shorter, warn
+  --min-conf-desc-length=n   set the minimum description length for config symbols
+                             in lines, if shorter, warn (default 4)

Include the minimum number of lines as other error messages already do:
- WARNING: please write a help paragraph that fully describes the config symbol
+ WARNING: please write a help paragraph that fully describes the config symbol with at least 4 lines

Link: https://lkml.kernel.org/r/c71c170c90eba26265951e248adfedd3245fe575.1741605695.git.p.hahn@avm.de
Signed-off-by: Philipp Hahn <p.hahn@avm.de>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Joe Perches <joe@perches.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>