Disambiguate argument descriptors from section headers (#9427)
## Summary
Given a docstring like:
```python
def func(x: int, args: tuple[int]):
"""Toggle the gizmo.
Args:
x: Some argument.
args: Some other arguments.
"""
```
We were considering the `args:` descriptor to be an indented docstring
section header (since `Args:`) is a valid header name. This led to very
confusing diagnostics.
This PR makes the parsing a bit more lax in this case, such that if we
see a nested header that's more deeply indented than the preceding
header, and the preceding section allows sub-items (like `Args:`), we
avoid treating the nested item as a section header.
Closes https://github.com/astral-sh/ruff/issues/9426.
