go.tools/cmd/vet: add stable checks to doc.go

These are the simplest possible descriptions of each command.
They may be fleshed out later.

Fixes golang/go#7298.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/61480044

cmd/vet/doc.go

@@ -9,6 +9,11 @@ calls whose arguments do not align with the format string. Vet uses heuristics
 that do not guarantee all reports are genuine problems, but it can find errors
 not caught by the compilers.
 
+Usage:
+
+	go tool vet [flag] [file.go ...]
+	go tool vet [flag] [directory ...] # Scan all .go files under directory, recursively
+
 Its exit code is 2 for erroneous invocation of the tool, 1 if a
 problem was reported, and 0 otherwise. Note that the tool does not
 check every possible problem and depends on unreliable heuristics
@@ -20,7 +25,9 @@ those identified by the flags are performed.
 
 Available checks:
 
-1. Printf family, flag -printf
+1. Printf family
+
+Flag -printf
 
 Suspicious calls to functions in the Printf family, including any functions
 with these names:
@@ -37,7 +44,9 @@ complains about arguments that look like format descriptor strings.
 It also checks for errors such as using a Writer as the first argument of
 Printf.
 
-2. Methods, flag -methods
+2. Methods
+
+Flag -methods
 
 Non-standard signatures for methods with familiar names, including:
 	Format GobEncode GobDecode MarshalJSON MarshalXML
@@ -45,21 +54,79 @@ Non-standard signatures for methods with familiar names, including:
 	UnmarshalJSON UnreadByte UnreadRune WriteByte
 	WriteTo
 
-3. Struct tags, flag -structtags
+3. Struct tags
+
+Flag -structtags
 
 Struct tags that do not follow the format understood by reflect.StructTag.Get.
 
-4. Unkeyed composite literals, flag -composites
+4. Unkeyed composite literals
+
+Flag -composites
 
 Composite struct literals that do not use the field-keyed syntax.
 
+5. Assembly declarations
 
-Usage:
+Flag -asmdecl
 
-	go tool vet [flag] [file.go ...]
-	go tool vet [flag] [directory ...] # Scan all .go files under directory, recursively
+Mismatches between assembly files and Go function declarations.
 
-The other flags are:
+6. Useless assignments
+
+Flag -assign
+
+Check for useless assignments.
+
+7. Atomic mistakes
+
+Flag -atomic
+
+Common mistaken usages of the sync/atomic package.
+
+8. Build tags
+
+Flag -buildtags
+
+Badly formed or misplaced +build tags.
+
+9. Copying locks
+
+Flag -copylocks
+
+Locks that are erroneously passed by value.
+
+10. Nil function comparison
+
+Flag -nilfunc
+
+Comparisons between functions and nil.
+
+11. Range loop variables
+
+Flag -rangeloops
+
+Incorrect uses of range loop variables in closures.
+
+12. Unreachable code
+
+Flag -unreachable
+
+Unreachable code.
+
+13. Shadowed variables
+
+Flag -shadow=false (experimental; must be set explicitly)
+
+Variables that may have been unintentionally shadowed.
+
+
+Other flags
+
+These flags configure the behavior of vet:
+
+	-all (default true)
+		Check everything; disabled if any explicit check is requested.
 	-v
 		Verbose mode
 	-printfuncs
@@ -71,6 +138,9 @@ The other flags are:
 		if you have Warn and Warnf functions that take an
 		io.Writer as their first argument, like Fprintf,
 			-printfuncs=Warn:1,Warnf:1
-
+	-shadowstrict
+		Whether to be strict about shadowing; can be noisy.
+	-test
+		For testing only: sets -all and -shadow.
 */
 package main