Add help text for dock property

sandbox/uber.css

@@ -7,6 +7,7 @@
 }
 
 .list-item {
+  dock: ac2 2;
   height: 8;
   min-width: 80;
   background: dark_blue;

src/textual/css/_help_text.py

@@ -352,3 +352,32 @@ def docks_property_help_text(
             ).get_by_context(context)
         ],
     )
+
+
+def dock_property_help_text(
+    property_name: str, context: StylingContext | None
+) -> HelpText:
+    property_name = _contextualize_property_name(property_name, context)
+    return HelpText(
+        summary=f"Invalid value for [i]{property_name}[/] property",
+        bullets=[
+            Bullet("The value must be one of the defined docks"),
+            *ContextSpecificBullets(
+                inline=[],
+                css=[
+                    Bullet(
+                        "Define a dock using the [i]docks[/] property",
+                        examples=[
+                            Example("docks: [u]lhs[/]=left/2;"),
+                        ],
+                    ),
+                    Bullet(
+                        "Then attach a widget to a defined dock using the [i]dock[/] property",
+                        examples=[
+                            Example("dock: [scope.key][u]lhs[/][/];"),
+                        ],
+                    ),
+                ],
+            ).get_by_context(context),
+        ],
+    )

src/textual/css/_styles_builder.py

@@ -15,6 +15,7 @@ from ._help_text import (
     border_property_help_text,
     layout_property_help_text,
     docks_property_help_text,
+    dock_property_help_text,
 )
 from .constants import (
     VALID_ALIGN_HORIZONTAL,
@@ -553,7 +554,7 @@ class StylesBuilder:
             self.error(
                 name,
                 tokens[1],
-                f"unexpected tokens in dock declaration",
+                dock_property_help_text(name, context="css"),
             )
         self.styles._rules["dock"] = tokens[0].value if tokens else ""