all: minor documentation tweaks for constants

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Change-Id: Iedfad67f4b8b9c84f128b98b9b06fa76919af388
Reviewed-on: https://go-review.googlesource.com/14357Reviewed-by: Rob Pike <r@golang.org>

src/go/types/type.go

@@ -360,7 +360,7 @@ type Chan struct {
 // A ChanDir value indicates a channel direction.
 type ChanDir int
 
-// The direction of a channel is indicated by one of the following constants.
+// The direction of a channel is indicated by one of these constants.
 const (
 	SendRecv ChanDir = iota
 	SendOnly

src/math/big/float.go

@@ -124,7 +124,7 @@ const (
 // rounding error is described by the Float's Accuracy.
 type RoundingMode byte
 
-// The following rounding modes are supported.
+// These constants define supported rounding modes.
 const (
 	ToNearestEven RoundingMode = iota // == IEEE 754-2008 roundTiesToEven
 	ToNearestAway                     // == IEEE 754-2008 roundTiesToAway

src/text/scanner/scanner.go

@@ -73,7 +73,7 @@ const (
 	GoTokens       = ScanIdents | ScanFloats | ScanChars | ScanStrings | ScanRawStrings | ScanComments | SkipComments
 )
 
-// The result of Scan is one of the following tokens or a Unicode character.
+// The result of Scan is one of these tokens or a Unicode character.
 const (
 	EOF = -(iota + 1)
 	Ident