vet: add -W flag and detailed help (#9331)

cmd/v/help/default.txt

@@ -49,4 +49,4 @@ Use "v help <command>" for more information about a command, example: `v help bu
 Use "v help other" to see less frequently used commands.
 
 Note: Help is required to write more help topics.
-Only build, new, init, doc, fmt, run, test, search, install, remove, update, bin2v, check-md are properly documented currently.
+Only build, new, init, doc, fmt, vet, run, test, search, install, remove, update, bin2v, check-md are properly documented currently.

cmd/v/help/vet.txt

@@ -0,0 +1,11 @@
+Usage:
+  v vet [options] path_to_source.v [path_to_other_source.v]
+  v vet [options] path/to/dir [path/to/other_dir]
+
+Reports suspicious code constructs.
+
+Options:
+  -W               Exit with code 1, even if vet only reported warnings. Useful for checks in CI.
+  -hide-warnings   Do not print warnings to stderr.
+  -v, -verbose     Enable verbose logging.
+  -force           (NB: vet development only!) Do not skip the vet regression tests.