DOC: specify return type in to_datetime
BUG: Option context applies on __enter__

Option context no longer overrides options when used outside a `with`
statement.

Added test TestConfig.test_option_config_scope

Closes #8514
Merge pull request #8921 from jorisvandenbossche/docstring-to_datetime

DOC: specify return type in to_datetime
Merge pull request #8925 from hkleynhans/fix/8514_option_context_with

BUG: Option context applies on __enter__