docs: clarify schema discovery for dbListTables

[LeonidasZhak]

Summary

Clarify how to discover tables in a non-default schema, both by listing objects with Id(schema = ...) and by using options = "-c search_path=..." for a connection.

Thanks to maintainers

Thanks for the pointers in #399 and for the existing schema-qualified table examples.

Issue or motivation

Issue #399 notes that users looking for tables in a specific schema do not easily find the supported workflows.

Root cause

The connection docs only pointed readers to the general libpq options reference, and the table docs showed schema-qualified writes without showing the matching schema-scoped listing patterns.

Change

• document options = "-c search_path=..." in the dbConnect() docs with a runnable example
• extend the table docs to show dbListObjects(con, Id(schema = ...)) and dbListTables() on a connection with search_path set
• add a focused test covering the documented search_path workflow

Tests

• devtools::document()
• UserNM=true R CMD INSTALL .
• devtools::test(filter = "dbConnect")
• rcmdcheck::rcmdcheck(args = c("--no-manual", "--ignore-vignettes"), error_on = "note")
• pkgdown::build_site(preview = FALSE)

Scope

I did not change dbListTables() behavior or add a new schema argument. This PR only documents the existing workflows and adds a focused regression test for the documented connection-option path.