ocamldoc: better support of multiline code blocks in the manpage backend

Before, `man Map` would show:

```
For instance: module IntPairs = struct type t = int * int let compare (x0,y0) (x1,y1) = match Stdlib.compare x0 x1 with 0 -> Stdlib.compare y0 y1 |
c -> c end module PairsMap = Map.Make(IntPairs) let m = PairsMap.(empty |> add (0,1) hello |> add (1,0) world )
```

Now it shows:

```
For instance:
     module IntPairs =
       struct
         type t = int * int
         let compare (x0,y0) (x1,y1) =
           match Stdlib.compare x0 x1 with
               0 -> Stdlib.compare y0 y1
             | c -> c
       end

     module PairsMap = Map.Make(IntPairs)

     let m = PairsMap.(empty |> add (0,1) "hello" |> add (1,0) "world")
```

(in both cases the code is in bold)

1 files changed, 7 insertions, 7 deletions

diff --git a/ocamldoc/odoc_man.ml b/ocamldoc/odoc_man.ml
index 3f4ddf4c02..f4ebaadea1 100644
--- a/ocamldoc/odoc_man.ml
+++ b/ocamldoc/odoc_man.ml
@@ -287,12 +287,8 @@ class man =
     method man_of_text_element b txt =
       match txt with
       | Odoc_info.Raw s -> bs b (self#escape s)
-      | Odoc_info.Code s ->
-          bs b "\n.B ";
-          bs b ((Str.global_replace (Str.regexp "\n") "\n.B " (self#escape s))^"\n")
-      | Odoc_info.CodePre s ->
-          bs b "\n.B ";
-          bs b ((Str.global_replace (Str.regexp "\n") "\n.B " (self#escape s))^"\n")
+      | Odoc_info.Code s -> self#man_of_code b s
+      | Odoc_info.CodePre s -> self#man_of_code b s
       | Odoc_info.Verbatim s ->
           bs b (self#escape s)
       | Odoc_info.Bold t
@@ -346,7 +342,11 @@ class man =
       if String.lowercase_ascii target = "man" then bs b code else ()
 
     (** Print groff string to display code. *)
-    method man_of_code b s = self#man_of_text b [ Code s ]
+    method man_of_code b code =
+      let code = self#escape code in
+      bs b "\n.ft B\n";
+      bs b (Str.global_replace (Str.regexp "\n") "\n.br\n\\&" code);
+      bs b "\n.ft R\n";
 
     (** Take a string and return the string where fully qualified idents
        have been replaced by idents relative to the given module name.*)