Avoid adding dashed line outside of docstring

[charliermarsh]

If you have a docstring like:

def no_underline_and_no_newline():  # noqa: D416
    """Toggle the gizmo.
    Returns"""

Then right now, when we inject the dashed underline under Returns, we end up putting it outside of the docstring 🤦 This PR adds handling for this (uncommon) "no following lines" case.

Closes #3534.

[github-actions]

PR Check Results

Ecosystem

✅ ecosystem check detected no changes.

Benchmark

Linux

group                                      main                                   pr
-----                                      ----                                   --
linter/all-rules/large/dataset.py          1.00     13.6±0.07ms     3.0 MB/sec  
linter/all-rules/numpy/ctypeslib.py        1.00      3.6±0.01ms     4.7 MB/sec  
linter/all-rules/numpy/globals.py          1.00    486.3±1.22µs     6.1 MB/sec  
linter/all-rules/pydantic/types.py         1.00      6.0±0.02ms     4.3 MB/sec  
linter/default-rules/large/dataset.py      1.00      7.6±0.02ms     5.4 MB/sec  
linter/default-rules/numpy/ctypeslib.py    1.00   1677.8±4.37µs     9.9 MB/sec  
linter/default-rules/numpy/globals.py      1.00    189.7±0.40µs    15.6 MB/sec  
linter/default-rules/pydantic/types.py     1.00      3.5±0.01ms     7.2 MB/sec  
linter/large/dataset.py                                                           1.00      7.6±0.03ms     5.3 MB/sec
linter/numpy/ctypeslib.py                                                         1.00      2.3±0.00ms   145.4 MB/sec
linter/numpy/globals.py                                                           1.00   1208.6±4.71µs   147.5 MB/sec
linter/pydantic/types.py                                                          1.00      3.6±0.01ms     7.2 MB/sec

Windows

group                                      main                                   pr
-----                                      ----                                   --
linter/all-rules/large/dataset.py          1.00     15.9±0.23ms     2.6 MB/sec  
linter/all-rules/numpy/ctypeslib.py        1.00      4.2±0.07ms     4.0 MB/sec  
linter/all-rules/numpy/globals.py          1.00   529.5±11.83µs     5.6 MB/sec  
linter/all-rules/pydantic/types.py         1.00      7.0±0.09ms     3.6 MB/sec  
linter/default-rules/large/dataset.py      1.00      8.8±0.13ms     4.6 MB/sec  
linter/default-rules/numpy/ctypeslib.py    1.00  1917.3±31.84µs     8.7 MB/sec  
linter/default-rules/numpy/globals.py      1.00    208.7±2.80µs    14.1 MB/sec  
linter/default-rules/pydantic/types.py     1.00      4.0±0.05ms     6.3 MB/sec  
linter/large/dataset.py                                                           1.00      8.9±0.25ms     4.6 MB/sec
linter/numpy/ctypeslib.py                                                         1.00      2.9±0.02ms   116.6 MB/sec
linter/numpy/globals.py                                                           1.00  1523.6±39.78µs   117.0 MB/sec
linter/pydantic/types.py                                                          1.00      4.1±0.09ms     6.2 MB/sec