The way that -all works was unclear from the documentation and made
worse by recent changes to the flag package. Improve matters by making
the help message say "default true" for the tests that do default to true,
and tweak some of the wording.

Before:

Usage of vet:
	vet [flags] directory...
	vet [flags] files... # Must be a single package
For more information run
	go doc cmd/vet

Flags:
  -all
    	enable all non-experimental checks (default unset)
  -asmdecl
    	check assembly against Go declarations (default unset)
...

After:

Usage of vet:
	vet [flags] directory...
	vet [flags] files... # Must be a single package
By default, -all is set and all non-experimental checks are run.
For more information run
	go doc cmd/vet

Flags:
  -all
    	enable all non-experimental checks (default true)
  -asmdecl
    	check assembly against Go declarations (default true)
...

Change-Id: Ie94b27381a9ad2382a10a7542a93bce1d59fa8f5
Reviewed-on: https://go-review.googlesource.com/21495Reviewed-by: Andrew Gerrand <adg@golang.org>