Tweak unknown annotation GDScript error for @deprecated/@experimental/@tutorial

These annotations don't exist at a source level, so the error messages
point to the documentation comment syntax.

modules/gdscript/tests/scripts/parser/errors/annotation_deprecated.gd

@@ -0,0 +1,5 @@
+# This annotation should be used within a documentation comment instead:
+# ## @deprecated: Reason here.
+
+@deprecated
+var some_variable = "value"

modules/gdscript/tests/scripts/parser/errors/annotation_deprecated.out

@@ -0,0 +1,2 @@
+GDTEST_PARSER_ERROR
+"@deprecated" annotation does not exist. Use "## @deprecated: Reason here." instead.

modules/gdscript/tests/scripts/parser/errors/annotation_experimental.gd

@@ -0,0 +1,6 @@
+# This annotation should be used within a documentation comment instead:
+# ## @experimental: Reason here.
+
+@experimental("This function isn't implemented yet.")
+func say_hello():
+    pass

modules/gdscript/tests/scripts/parser/errors/annotation_experimental.out

@@ -0,0 +1,2 @@
+GDTEST_PARSER_ERROR
+"@experimental" annotation does not exist. Use "## @experimental: Reason here." instead.

modules/gdscript/tests/scripts/parser/errors/annotation_tutorial.gd

@@ -0,0 +1,5 @@
+# This annotation should be used within a documentation comment instead:
+# ## @tutorial(Title): https://example.com
+
+@tutorial("https://example.com")
+const SOME_CONSTANT = "value"

modules/gdscript/tests/scripts/parser/errors/annotation_tutorial.out

@@ -0,0 +1,2 @@
+GDTEST_PARSER_ERROR
+"@tutorial" annotation does not exist. Use "## @tutorial(Title): https://example.com" instead.