Ignore underlines when determining docstring logical lines (#8929)

crates/ruff_linter/src/rules/pydocstyle/helpers.rs

@@ -10,8 +10,9 @@ pub(super) fn logical_line(content: &str) -> Option<usize> {
     // Find the first logical line.
     let mut logical_line = None;
     for (i, line) in content.universal_newlines().enumerate() {
-        if line.trim().is_empty() {
-            // Empty line. If this is the line _after_ the first logical line, stop.
+        let trimmed = line.trim();
+        if trimmed.is_empty() || trimmed.chars().all(|c| matches!(c, '-' | '~' | '=' | '#')) {
+            // Empty line, or underline. If this is the line _after_ the first logical line, stop.
             if logical_line.is_some() {
                 break;
             }

crates/ruff_linter/src/rules/pydocstyle/snapshots/ruff_linter__rules__pydocstyle__tests__D400_D400.py.snap

@@ -247,4 +247,28 @@ D400.py:69:5: D400 [*] First line should end with a period
 73 73 | 
 74 74 | 
 
+D400.py:97:5: D400 [*] First line should end with a period
+    |
+ 96 |   def f():
+ 97 |       """
+    |  _____^
+ 98 | |     My example
+ 99 | |     ==========
+100 | | 
+101 | |     My example explanation
+102 | |     """
+    | |_______^ D400
+    |
+    = help: Add period
+
+ℹ Unsafe fix
+95 95 | 
+96 96 | def f():
+97 97 |     """
+98    |-    My example
+   98 |+    My example.
+99 99 |     ==========
+100 100 | 
+101 101 |     My example explanation
+