database/sql: add description to String method of IsolationLevel struct.

Add simple description to String method of IsolationLevel struct.

Change-Id: I8bdf829c81d4895b8542a3f21437bed61c6e925d
GitHub-Last-Rev: 7e16d6a93a9560cf3a09413dfc47d0cb04bb2fde
GitHub-Pull-Request: golang/go#28560
Reviewed-on: https://go-review.googlesource.com/c/147040


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -133,6 +133,7 @@ const (
 	LevelLinearizable
 )
 
+// String returns the name of the transaction isolation level.
 func (i IsolationLevel) String() string {
 	switch i {
 	case LevelDefault: