Add Libdl.dlext to documentation

[jayschwa]

Prompted by https://groups.google.com/d/topic/julia-dev/Xk3occIColg/discussion

[stevengj]

I think you actually have to add this as a docstring to libdl.jl ... the .rst file is generated from that (though a .. data:: dlext entry in the .rst file is required as a placeholder so that the doc-generation system knows where to put the docstring.

[jayschwa]

Hmm, okay. I didn't see other strings from this .rst file in the corresponding .jl file. If true, https://github.com/JuliaLang/julia/blob/master/CONTRIBUTING.md#improving-documentation should probably make mention of it.

[stevengj]

Most of the docs are still in base/docs/helpdb.jl, but that is slowly changing. Probably the CONTRIBUTING file needs to be updated on the new doc procedure, which changed very recently.

cc: @one-more-minute

[jakebolewski]

@jayschwa see CONTRIBUTING.md for how to do doc additions now.