tools/cmd/vet: Sort checks list alphabetically.
tools/cmd/vet: Sort checks list alphabetically.

The existing list of checks in doc.go has become somewhat unwieldy.
This commit is purely cosmetic and sorts the list alphabetically keyed
by the flag name.

Change-Id: Ibb56357f9d4667d199acc6637dc28f6e17903046
Reviewed-on: https://go-review.googlesource.com/14558
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

golang/tools,golang/tools,jdhenke/tools,syreclabs/go-tools,syreclabs/go-tools,magiccao/tools,golang/tools,syreclabs/go-tools,golang/tools,magiccao/tools,cmars/tools,jdhenke/tools,cmars/tools,jdhenke/tools,magiccao/tools,magiccao/tools,cmars/tools,golang/tools,syreclabs/go-tools,cmars/tools,jdhenke/tools,syreclabs/go-tools,syreclabs/go-tools