Add godoc strings to all exported types, functions, and fields

Documents:
- cli: NO_COLOR, RunOptions fields, CLIApp, VersionString
- checker: Result fields, Status constants, CheckStatus fields
- scanner: EnumerateStatus, ScanStatus, Options, FileEntry fields
- log: Level alias, DisableStyling, Init, Info/Debug functions,
  verbosity helpers, GetLogger, GetLevel, WithError
- mfer: ManifestScanOptions, New, NewFromPaths, NewFromFS, MAGIC

mfer/serialize.go

@@ -10,9 +10,7 @@ import (
 	"google.golang.org/protobuf/proto"
 )
 
-// was go-generate protoc --go_out=. --go_opt=paths=source_relative mf.proto
-
-// rot13("MANIFEST")
+// MAGIC is the file format magic bytes prefix (rot13 of "MANIFEST").
 const MAGIC string = "ZNAVSRFG"
 
 func newTimestampFromTime(t time.Time) *Timestamp {