Improve docstrings

Author: darrenburns

src/textual/widgets/_text_area.py

@@ -1835,6 +1835,8 @@ def action_cursor_left(self, select: bool = False) -> None:
         If the cursor is at the left edge of the document, try to move it to
         the end of the previous line.
 
+        If text is selected, move the cursor to the start of the selection.
+
         Args:
             select: If True, select the text while moving.
         """
@@ -1858,6 +1860,8 @@ def action_cursor_right(self, select: bool = False) -> None:
 
         If the cursor is at the end of a line, attempt to go to the start of the next line.
 
+        If text is selected, move the cursor to the end of the selection.
+
         Args:
             select: If True, select the text while moving.
         """