Backport of E2E: clarify drain -deadline and -force flag behaviors into release/1.4.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #16868 to be assessed for backporting due to the inclusion of the label backport/1.4.x.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

The below text is copied from the body of the original PR.

---

The -deadline and -force flag for the nomad node drain command only cause the draining to ignore the migrate block's healthy deadline, max parallel, etc. These flags don't have anything to do with the kill_timeout or shutdown_delay options of the jobspec.

This changeset fixes the skipped E2E tests so that they validate the intended behavior, and updates the docs for more clarity.

Extracted from #14348
Fixes #9902

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$ go test -v -count=1 ./nodedrain -run TestNodeDrain
=== RUN   TestNodeDrain
=== RUN   TestNodeDrain/IgnoreSystem
    node_drain_test.go:394: alloc has drained from node=168ea537 after 17.504451661s
=== RUN   TestNodeDrain/EphemeralMigrate
    node_drain_test.go:394: alloc has drained from node=e85597df after 17.014548575s
=== RUN   TestNodeDrain/KeepIneligible
=== RUN   TestNodeDrain/KillTimeout
    node_drain_test.go:204: draining node 4a75dad8-c862-784d-7bc6-bb6325b2b41d
    node_drain_test.go:215: waiting for kill_timeout to expire
    node_drain_test.go:228: waiting for migration to complete
    node_drain_test.go:394: alloc has drained from node=4a75dad8 after 17.018613647s
=== RUN   TestNodeDrain/DeadlineFlag
    node_drain_test.go:261: draining nodes e85597df-2f1c-ae83-168b-d4d8719a47a6, e85597df-2f1c-ae83-168b-d4d8719a47a6
    node_drain_test.go:284: waiting for old allocs to stop
    node_drain_test.go:287: waiting for running allocs
=== RUN   TestNodeDrain/ForceFlag
    node_drain_test.go:308: draining nodes 4a75dad8-c862-784d-7bc6-bb6325b2b41d, 168ea537-1b9e-7ec8-b2c2-53064ce427d8
    node_drain_test.go:329: waiting for old allocs to stop
    node_drain_test.go:332: waiting for running allocs
--- PASS: TestNodeDrain (112.08s)
    --- PASS: TestNodeDrain/IgnoreSystem (22.05s)
    --- PASS: TestNodeDrain/EphemeralMigrate (21.46s)
    --- PASS: TestNodeDrain/KeepIneligible (1.26s)
    --- PASS: TestNodeDrain/KillTimeout (44.69s)
    --- PASS: TestNodeDrain/DeadlineFlag (12.01s)
    --- PASS: TestNodeDrain/ForceFlag (10.42s)
PASS
ok      github.com/hashicorp/nomad/e2e/nodedrain        112.084s

[hashicorp-cla]

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[tgross]

Backports #16823 #16826 (E2E changes only) and #16868