docs: correct concurrency limits, error group alerts, TTL vs delay, and pending version

Four docs corrections, one per commit.

**Concurrency limits** (`limits.mdx`) — the table published a single
headline figure per tier, flattened from the four per-environment values
a plan actually sets, and drifted from the current defaults. Now split
per environment, with a note that the dashboard Concurrency page is
authoritative because the enforced limit also includes purchased add-on
and manual concurrency.

**Error group alerts** (`troubleshooting-alerts.mdx`) — undocumented
until now. They are created from the Errors page, appear in the Alerts
table, and are issue-based: they fire on a new issue, a regression or an
unignore, and a group that has alerted latches to Unresolved and stays
quiet. People configure one expecting a notification per failed run and
get silence. Documents the firing rules, the quiet steady state, and
which alert type to pick for "tell me about every failed run". Also adds
the `alert.error` webhook, which the reference omitted.

**Maximum run TTL** (`limits.mdx`) — "all runs have an enforced maximum
TTL of 14 days" reads as a cap on how far ahead you can schedule. It
isn't: a delayed run arms no TTL expiry, and the TTL clock starts when
the delay elapses. Also states that `delay` has no upper bound.

**Pending version** (`troubleshooting.mdx`) — a new troubleshooting
entry. A run parks here when the version deployed to that environment
carries neither the run's task nor its queue. Lists the four causes seen
in support: task missing from the deployed version, a queue no deployed
task declares, triggering into the wrong org/project/environment, and a
later deploy removing or renaming the task or queue while the run was
waiting. Also covers runs parked by version skew protection, which wait
for their specific external deployment id and expire after 1 hour.

Docs only.

Mono-RevId: c8a80f8c46ff160ac72fc7c1a1ef5f88a0b02f2c