database/sql/driver: document that Execer, Queryer need not be implemented

CL 21663 allowed drivers to implement ExecerContext without
also implementing Execer, and similarly QueryerContext without
Queryer, but it did not make that clear in the documentation.

This CL updates the documentation.

Change-Id: I9a4accaac32edfe255fe7c0b0907d4c1014322b4
Reviewed-on: https://go-review.googlesource.com/78129Reviewed-by: Daniel Theophanes <kardianos@gmail.com>

src/database/sql/driver/driver.go

@@ -109,22 +109,23 @@ type Pinger interface {
 
 // Execer is an optional interface that may be implemented by a Conn.
 //
-// If a Conn does not implement Execer, the sql package's DB.Exec will
-// first prepare a query, execute the statement, and then close the
-// statement.
+// If a Conn implements neither ExecerContext nor Execer Execer,
+// the sql package's DB.Exec will first prepare a query, execute the statement,
+// and then close the statement.
 //
 // Exec may return ErrSkip.
 //
-// Deprecated: Drivers should implement ExecerContext instead (or additionally).
+// Deprecated: Drivers should implement ExecerContext instead.
 type Execer interface {
 	Exec(query string, args []Value) (Result, error)
 }
 
 // ExecerContext is an optional interface that may be implemented by a Conn.
 //
-// If a Conn does not implement ExecerContext, the sql package's DB.Exec will
-// first prepare a query, execute the statement, and then close the
-// statement.
+// If a Conn does not implement ExecerContext, the sql package's DB.Exec
+// will fall back to Execer; if the Conn does not implement Execer either,
+// DB.Exec will first prepare a query, execute the statement, and then
+// close the statement.
 //
 // ExecerContext may return ErrSkip.
 //
@@ -135,22 +136,23 @@ type ExecerContext interface {
 
 // Queryer is an optional interface that may be implemented by a Conn.
 //
-// If a Conn does not implement Queryer, the sql package's DB.Query will
-// first prepare a query, execute the statement, and then close the
-// statement.
+// If a Conn implements neither QueryerContext nor Queryer,
+// the sql package's DB.Query will first prepare a query, execute the statement,
+// and then close the statement.
 //
 // Query may return ErrSkip.
 //
-// Deprecated: Drivers should implement QueryerContext instead (or additionally).
+// Deprecated: Drivers should implement QueryerContext instead.
 type Queryer interface {
 	Query(query string, args []Value) (Rows, error)
 }
 
 // QueryerContext is an optional interface that may be implemented by a Conn.
 //
-// If a Conn does not implement QueryerContext, the sql package's DB.Query will
-// first prepare a query, execute the statement, and then close the
-// statement.
+// If a Conn does not implement QueryerContext, the sql package's DB.Query
+// will fall back to Queryer; if the Conn does not implement Queryer either,
+// DB.Query will first prepare a query, execute the statement, and then
+// close the statement.
 //
 // QueryerContext may return ErrSkip.
 //