database/sql: document Connect and Close may need a timeout

Opening a connection with Connect should still create a derived
context with a timeout because some clients will not use a timeout
and the connection pool may open a connection asynchronously.

Likewise, if a connection close makes a network operation it should
provide some type of sane timeout for the operation.

Fixes #38185

Change-Id: I9b7ce2996c81c486170dcc84b12672a99610fa27
Reviewed-on: https://go-review.googlesource.com/c/go/+/230438


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>