[docs-patrol] Update Heartbeat docs for new `api` monitor type and scheduler limit env var

Recent code changes in the repository have introduced documentation drift. The following change needs corresponding documentation updates.

## Changes Requiring Documentation Updates

### 1. Heartbeat adds `monitor.type: api`, but monitor-type and scheduler docs still omit it

**Commit(s):**
- `90c9524e3136b825bfc9a7a060ef64d78a0f11f0` — https://github.com/elastic/beats/commit/90c9524e3136b825bfc9a7a060ef64d78a0f11f0

**What changed:**
- New monitor type registration in `x-pack/heartbeat/monitors/api/api.go:6` and `x-pack/heartbeat/monitors/api/api.go:28`.
- Job limit defaults/env handling include `api` in `heartbeat/config/config.go:71` and `heartbeat/config/config.go:77` (including `SYNTHETICS_LIMIT_API`).

**Documentation impact:**
- `docs/reference/heartbeat/configuration-heartbeat-options.md:75-86` lists monitor types as only `icmp`, `tcp`, and `http`.
- `docs/reference/heartbeat/monitors-scheduler.md:52` says `SYNTHETICS_LIMIT_{{TYPE}}` supports only `HTTP`, `TCP`, and `ICMP`.
- `x-pack/heartbeat/heartbeat.reference.yml:257-266` documents job limits for `browser/http/tcp/icmp` but not `api`.

These docs are now out of date relative to the current behavior and configuration surface.

## Suggested Actions

- [ ] Add `api` monitor type coverage to `docs/reference/heartbeat/configuration-heartbeat-options.md` (including scope/usage expectations).
- [ ] Update `docs/reference/heartbeat/monitors-scheduler.md` to include `API` in documented `SYNTHETICS_LIMIT_{{TYPE}}` values (and align with currently supported monitor types).
- [ ] Add `api` example/commented key to `x-pack/heartbeat/heartbeat.reference.yml` under `heartbeat.jobs`.

Duplicate check: no matching open docs-tracking issue was found in repository issue/PR search for `SYNTHETICS_LIMIT_API` or `monitor.type: api`, and this does not match entries in `/tmp/previous-findings.json`.

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Start with the existing monitor-type list in docs/reference/heartbeat/configuration-heartbeat-options.md and the scheduler limit section in docs/reference/heartbeat/monitors-scheduler.md, then compare them with x-pack/heartbeat/heartbeat.reference.yml and the cited Heartbeat config and API monitor code. Update all three documentation locations to cover api and SYNTHETICS_LIMIT_API, including the requested example and usage details; done means the documented options match the current implementation.

Written by the indexing model from the issue text.