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.

crates/ruff_linter/src/rules/pydocstyle/snapshots/ruff_linter__rules__pydocstyle__tests__D214_sections.py.snap

@@ -27,4 +27,27 @@ sections.py:144:5: D214 [*] Section is over-indented ("Returns")
 148 148 |     A value of some sort.
 149 149 | 
 
+sections.py:558:5: D214 [*] Section is over-indented ("Returns")
+    |
+557 |   def titlecase_sub_section_header():
+558 |       """Below, `Returns:` should be considered a section header.
+    |  _____^
+559 | | 
+560 | |     Args:
+561 | |         Here's a note.
+562 | | 
+563 | |         Returns:
+564 | |     """
+    | |_______^ D214
+    |
+    = help: Remove over-indentation from "Returns"
+
+ℹ Safe fix
+560 560 |     Args:
+561 561 |         Here's a note.
+562 562 | 
+563     |-        Returns:
+    563 |+    Returns:
+564 564 |     """
+
 

crates/ruff_linter/src/rules/pydocstyle/snapshots/ruff_linter__rules__pydocstyle__tests__D407_sections.py.snap

@@ -498,4 +498,74 @@ sections.py:527:5: D407 [*] Missing dashed underline after section ("Parameters"
 531 532 |     """
 532 533 | 
 
+sections.py:548:5: D407 [*] Missing dashed underline after section ("Args")
+    |
+547 |   def lowercase_sub_section_header():
+548 |       """Below, `returns:` should _not_ be considered a section header.
+    |  _____^
+549 | | 
+550 | |     Args:
+551 | |         Here's a note.
+552 | | 
+553 | |         returns:
+554 | |     """
+    | |_______^ D407
+    |
+    = help: Add dashed line under "Args"
+
+ℹ Safe fix
+548 548 |     """Below, `returns:` should _not_ be considered a section header.
+549 549 | 
+550 550 |     Args:
+    551 |+    ----
+551 552 |         Here's a note.
+552 553 | 
+553 554 |         returns:
+
+sections.py:558:5: D407 [*] Missing dashed underline after section ("Args")
+    |
+557 |   def titlecase_sub_section_header():
+558 |       """Below, `Returns:` should be considered a section header.
+    |  _____^
+559 | | 
+560 | |     Args:
+561 | |         Here's a note.
+562 | | 
+563 | |         Returns:
+564 | |     """
+    | |_______^ D407
+    |
+    = help: Add dashed line under "Args"
+
+ℹ Safe fix
+558 558 |     """Below, `Returns:` should be considered a section header.
+559 559 | 
+560 560 |     Args:
+    561 |+    ----
+561 562 |         Here's a note.
+562 563 | 
+563 564 |         Returns:
+
+sections.py:558:5: D407 [*] Missing dashed underline after section ("Returns")
+    |
+557 |   def titlecase_sub_section_header():
+558 |       """Below, `Returns:` should be considered a section header.
+    |  _____^
+559 | | 
+560 | |     Args:
+561 | |         Here's a note.
+562 | | 
+563 | |         Returns:
+564 | |     """
+    | |_______^ D407
+    |
+    = help: Add dashed line under "Returns"
+
+ℹ Safe fix
+561 561 |         Here's a note.
+562 562 | 
+563 563 |         Returns:
+    564 |+    -------
+564 565 |     """
+
 

crates/ruff_linter/src/rules/pydocstyle/snapshots/ruff_linter__rules__pydocstyle__tests__D414_sections.py.snap

@@ -97,4 +97,18 @@ sections.py:261:5: D414 Section has no content ("Returns")
     | |_______^ D414
     |
 
+sections.py:558:5: D414 Section has no content ("Returns")
+    |
+557 |   def titlecase_sub_section_header():
+558 |       """Below, `Returns:` should be considered a section header.
+    |  _____^
+559 | | 
+560 | |     Args:
+561 | |         Here's a note.
+562 | | 
+563 | |         Returns:
+564 | |     """
+    | |_______^ D414
+    |
+
 

crates/ruff_linter/src/rules/pydocstyle/snapshots/ruff_linter__rules__pydocstyle__tests__d417_google.snap

@@ -64,4 +64,12 @@ D417.py:108:5: D417 Missing argument description in the docstring for `f`: `*arg
 109 |     """Do something.
     |
 
+D417.py:155:5: D417 Missing argument description in the docstring for `select_data`: `auto_save`
+    |
+155 | def select_data(
+    |     ^^^^^^^^^^^ D417
+156 |     query: str,
+157 |     args: tuple,
+    |
+
 

crates/ruff_linter/src/rules/pydocstyle/snapshots/ruff_linter__rules__pydocstyle__tests__d417_unspecified.snap

@@ -64,4 +64,12 @@ D417.py:108:5: D417 Missing argument description in the docstring for `f`: `*arg
 109 |     """Do something.
     |
 
+D417.py:155:5: D417 Missing argument description in the docstring for `select_data`: `auto_save`
+    |
+155 | def select_data(
+    |     ^^^^^^^^^^^ D417
+156 |     query: str,
+157 |     args: tuple,
+    |
+