database/sql: update minor sql docs

Replace the work "session" with "connection" in docs. Fix
The ErrConnDone documentation. Clarify what the context is used
for in StmtContext.

Change-Id: I2f07e58d0cd6321b386a73b038cf6070cb8e2572
Reviewed-on: https://go-review.googlesource.com/65732Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/database/sql/sql.go

@@ -1492,7 +1492,7 @@ func (db *DB) Driver() driver.Driver {
 }
 
 // ErrConnDone is returned by any operation that is performed on a connection
-// that has already been committed or rolled back.
+// that has already been returned to the connection pool.
 var ErrConnDone = errors.New("database/sql: connection is already closed")
 
 // Conn returns a single connection by either opening a new connection
@@ -1527,9 +1527,9 @@ func (db *DB) Conn(ctx context.Context) (*Conn, error) {
 
 type releaseConn func(error)
 
-// Conn represents a single database session rather than a pool of database
-// sessions. Prefer running queries from DB unless there is a specific
-// need for a continuous single database session.
+// Conn represents a single database connection rather than a pool of database
+// connections. Prefer running queries from DB unless there is a specific
+// need for a continuous single database connection.
 //
 // A Conn must call Close to return the connection to the database pool
 // and may do so concurrently with a running query.
@@ -1893,6 +1893,9 @@ func (tx *Tx) Prepare(query string) (*Stmt, error) {
 //  ...
 //  res, err := tx.StmtContext(ctx, updateMoney).Exec(123.45, 98293203)
 //
+// The provided context is used for the preparation of the statement, not for the
+// execution of the statement.
+//
 // The returned statement operates within the transaction and will be closed
 // when the transaction has been committed or rolled back.
 func (tx *Tx) StmtContext(ctx context.Context, stmt *Stmt) *Stmt {