Document context use of tds.NewConn and do not pass context from data…

[ntnn]

…base/sql call

Description

If a user passes a timeout context to database/sql.Open* the connection will close after the timeout finishes or is cancelled. For tds.NewConn this behaviour is desired, for purego.NewConn it is not.

Related issues

Link any related issues here.

Tests

• [x] make lint
• [x] make test-go / make test-cgo
• [x] make integration-go / make integration-cgo