Update tools.mk to 0.5.6
Fix edoc error in dvvset.erl

Remove use of `@title` tag in module.

From the [Edoc User's Guide
0.7.13](http://www.erlang.org/doc/apps/edoc/chapter.html): Specifies a
title for the overview page. This tag can _only_ be used in an overview
file. The content can be arbitrary text.
Fix `` quoting to `' for Edoc
Don't use @doc tag for -callback

Edoc complains and fails to build the file. Unfortunately, putting the
comment below or above the `-callback` doesn't get it to show up in the
html. But this at least allows it to succeed.
Clean up @see and {@link foo/1} edoc usage

EDoc doesn't seem to like the `{@see foo/1}` syntax within a larger `@doc`.
I've only gotten `@see` to work as the first bit of a comment line:

```erlang
%% @doc foo bar baz
%% @see foo/1
```

Many of the inline uses I was able to simply change to use `{@link
foo/2}`.

Edoc now successfully completes with no errors or warnings.
disable warnings_as_errors