Bump EdgeCacheOrigin timeout + add reference docs

[rileykarson]

Took 81m to fail in a test.

Release Note Template for Downstream PRs (will be copied)

networkservices: increased default timeout for `google_network_services_edge_cache_origin` to 120m from 60m

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 2 files changed, 9 insertions(+), 6 deletions(-))
Terraform Beta: Diff ( 2 files changed, 9 insertions(+), 6 deletions(-))

[modular-magician]

Tests analytics

Total tests: 3044
Passed tests 2746
Skipped tests: 298
Affected tests: 0

$\textcolor{green}{\textsf{All tests passed in REPLAYING mode.}}$
View the build log

[zli82016]

LGTM. Thanks.