Enable tail call optimization in couch_replicator_docs

[nickva]

update_docs/4 function was recursively called from catch part of a
try ... catch block. That construct prevented tail call optimization to
take place.

Jira: COUCHDB-3324

[nickva]

@davisp pointed out that current code already has TCO and this is not needed

Used this script from him (with a modification to just print the memory of the process) to verify:

-module(foo).
-compile(export_all).
a() ->
    a(0).
a(I) ->
    if (I+1) rem 1000 /= 0 -> ok; true ->
        io:format("~p~n", [process_info(self(), memory)])
    end,
    try
        throw(bar)
    catch throw:bar ->
        c(a, I+1)
    end.
b() ->
    b(0).
b(I) ->
    if (I+1) rem 1000 /= 0 -> ok; true ->
        io:format("~p~n", [process_info(self(), memory)])
    end,
    try
        c(b, I+1)
    catch throw:bar ->
        foo
    end.
c(F, I) ->
    ?MODULE:F(I).

foo:b() ends up producing

...
{memory,102684144}
{memory,102684144}
{memory,102684144}
{memory,102684144}

foo:a() is stable

...
{memory,16672}
{memory,24576}
{memory,11784}
{memory,16672}

[iilyak]

Thank you Paul and Nick for pointing out that I was wrong. The code would work as is. @nickva could you close the PR?

[nickva]

@iilyak ah no worries, I assumed it needed TCO as well, so I think it was a useful exercise to understand how it works.