add a test to show issue ocaml/ocaml-lsp#344

tests/test-dirs/random-doc-issue.t

@@ -0,0 +1,78 @@
+Reproduce [ocaml-lsp #344](https://github.com/ocaml/ocaml-lsp/issues/344): a symbol's
+documentation is some other random documentation from a library that was last pulled for
+completion. See the issue for details. The space is necessary for the position of
+documentation to be fetch-able.
+
+  $ cat >lib_doc.ml <<EOF
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > 
+  > let k = ()
+  > let m = List.map
+  > EOF
+
+we need merlin to keep state between requests, so using server
+
+  $ $MERLIN server stop-server
+
+see that there is no doc for k
+
+  $ $MERLIN server document -position 23:5 < lib_doc.ml | jq '.value'
+  "No documentation available"
+
+we trigger the bug
+
+  $ $MERLIN server document -position 24:15 -filename lib_doc < lib_doc.ml
+  {
+    "class": "return",
+    "value": " [map f [a1; ...; an]] applies function [f] to [a1, ..., an],
+     and builds the list [[f a1; ...; f an]]
+     with the results returned by [f]. Not tail-recursive.
+   ",
+    "notifications": []
+  }
+
+random documentation is fetched for the same `document` request as before
+
+  $ $MERLIN server document -position 23:5 < lib_doc.ml
+  {
+    "class": "return",
+    "value": "List operations.
+  
+     Some functions are flagged as not tail-recursive.  A tail-recursive
+     function uses constant stack space, while a non-tail-recursive function
+     uses stack space proportional to the length of its list argument, which
+     can be a problem with very long lists.  When the function takes several
+     list arguments, an approximate formula giving stack usage (in some
+     unspecified constant unit) is shown in parentheses.
+  
+     The above considerations can usually be ignored if your lists are not
+     longer than about 10000 elements.
+  
+     The labeled version of this module can be used as described in the
+     {!StdLabels} module.",
+    "notifications": []
+  }
+
+stop server
+
+  $ $MERLIN server stop-server