document: add a test showing retrieval of docstrings not present on AST

Author: trefis

tests/test-dirs/src-documentation.t/dune renamed to tests/test-dirs/document/src-documentation.t/dune

@@ -0,0 +1,33 @@
+A test showing that we manage to get docstrings even when they are not kept as
+attributes on the AST.
+
+  $ cat >test.ml <<EOF
+  > let foo x y = (** incorrect doc for foo *)
+  >   x + y
+  >  
+  > let bar = foo
+  > EOF
+
+  $ $MERLIN single document -position 4:13 -filename test.ml < test.ml
+  {
+    "class": "return",
+    "value": "incorrect doc for foo",
+    "notifications": []
+  }
+
+And that it also works outside of the current buffer:
+
+  $ $OCAMLC -c -bin-annot -w +50 test.ml
+  File "test.ml", line 1, characters 14-42:
+  1 | let foo x y = (** incorrect doc for foo *)
+                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  Warning 50 [unexpected-docstring]: unattached documentation comment (ignored)
+
+  $ $MERLIN single document -position 1:18 -filename outside.ml << EOF
+  > let bar = Test.foo
+  > EOF
+  {
+    "class": "return",
+    "value": "incorrect doc for foo",
+    "notifications": []
+  }