`docments(full=True)` drops `**kwargs` docments unless `args_kwargs=True` is passed explicitly

`docments(..., full=True)` does not retain a trailing docment attached to `**kwargs`.

This contradicts the documentation in `04_docments.ipynb`, which says that `args_kwargs=True` is always enabled when `full=True`:
> ... (Note that for `full`, `args_kwargs=True` is always set too.)

## Reproduction

```python
from fastcore.docments import docments

def f(
**kwargs, # keyword arguments
): pass

docments(f, full=True)['kwargs']['docment']
```

Actual result:

```python
None
```

Expected result:

```python
'keyword arguments'
```

Passing `args_kwargs=True` explicitly produces the expected result:

```python
docments(f, full=True, args_kwargs=True)['kwargs']['docment']
# 'keyword arguments'
```

## Environment

- fastcore: 2.2.13
- Branch: `main`

## Likely cause

`docments()` passes `args_kwargs` unchanged to `_param_locs()`, even when `full=True`:

```python
p = _param_locs(s, returns=returns, args_kwargs=args_kwargs) or {}
```

This appears inconsistent with the documented behavior that full docments include variadic parameter docments.

A possible fix is to make `full=True` imply `args_kwargs=True`.

Start by reading docments() and _param_locs(), then run the supplied reproduction for a function with a trailing **kwargs docment. Check that full=True follows the documented behavior in 04_docments.ipynb and returns 'keyword arguments' without explicitly passing args_kwargs=True; preserve coverage for the explicit option.

Written by the indexing model from the issue text.