Resurrect ApplyLeft and ApplyRight's documentation

[PetarKirov]

The issue was that PR #4374 introduced a private symbol (SmartAlias) between ApplyLeft and ApplyRight's and they're ddoc-ed unittests. Since ddoc-ed unittests bind to first symbol above them and that symbol was private, they were omitted from documentation.

Also put the signature of isImplicitlyConvertible in a comment to make the example more clear.

[PetarKirov]

Ping @MartinNowak I'm targeting the stable branch because this a trivial change (just moving stuff around) and because I don't want to wait several months for the non-prerelease documentation to be updated.

[PetarKirov]

For comparison: before and after.

[MartinNowak]

Ping @MartinNowak I'm targeting the stable branch because this a trivial change (just moving stuff around) and because I don't want to wait several months for the non-prerelease documentation to be updated.

Our release cycle is supposed to be 2 month. The last one was an exception due to the big import/lookup change. So impatience is not argument for addressing stable, but bugfixes (like this PR) should always go into stable.

[PetarKirov]

Thanks!