Update /api/v3/write_lp precision parameter docs for additional short forms

## Description

Update the `/api/v3/write_lp` precision parameter documentation to include additional short-form values that are supported in the latest Enterprise build but not yet released in Core/Enterprise.

## Additional precision short forms to document

As of the latest Enterprise build, the following additional short forms are supported:

- `u` (in addition to `us`) for microsecond precision
- `n` (in addition to `ns`) for nanosecond precision

Reference code from `influxdb3_types/src/write.rs`:
```rust
match precision {
"auto" => Self::Auto,
"s" | "second" => Self::Second,
"ms" | "millisecond" => Self::Millisecond,
"us" | "u" | "microsecond" => Self::Microsecond,
"ns" | "n" | "nanosecond" => Self::Nanosecond,
_ => return Err(format!("unrecognized precision unit: {precision}")),
}
```

## Files to update

1. **`content/shared/influxdb3-write-guides/http-api/v3-write-lp.md`**
- Add `u` and `n` to the precision values list
- Update examples if needed

2. **`api-docs/influxdb3/core/v3/ref.yml`**
- Add `u` and `n` to the `PrecisionWrite` schema enum
- Update description to list both forms

3. **`api-docs/influxdb3/enterprise/v3/ref.yml`**
- Add `u` and `n` to the `PrecisionWrite` schema enum
- Update description to list both forms

## When to update

Wait until the next Core/Enterprise release that includes this change. The feature is currently in the Enterprise latest build but not yet in a stable release.

## Related

- Related to precision parameter documentation added in commit 402451c7c

Start with content/shared/influxdb3-write-guides/http-api/v3-write-lp.md and compare its precision values with the PrecisionWrite schemas in api-docs/influxdb3/core/v3/ref.yml and api-docs/influxdb3/enterprise/v3/ref.yml. After the relevant Core/Enterprise release, done means all three files document and enumerate u and n alongside the existing short forms, with examples updated if needed.

Written by the indexing model from the issue text.