database/sql/driver: document Valuer must not panic

While it is possible for the connection pool to guard against panics
on every valuer read, this seems like a high cost to be added,
in both code complexity and possible runtime performance.

Most uses of the Valuer will be trivial, like returning
a struct field. Optimize for that case. If sometime may panic the
valuer should itself use recover and return an error.

Fixes #26332

Change-Id: Iad18780b8028f669f5a7841b74a5384d62fb6a7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/170700


Reviewed-by: Ian Lance Taylor <iant@golang.org>