declare that "///" is still a doc comment, just not "////+" (fixes #5838)
auto merge of #6680 : ben0x539/rust/slashslashslash, r=graydon

There's currently a function in the lexer that rejects a line comment that is all slashes from being a doc comment. I think the intention was that you could draw boxes,

    /////////////
    // like so //
    /////////////

Since a line doc comment split up over multiple paragraphs will have a "blank" line that is just /// between the paragraphs, that would get mistaken for a box segment, lexed as a regular comment, and go missing from the sequence of doc comment attributes before they were reassembled by rustdoc into markdown input.

I figure the best plan here is to just declare that a comment that is exactly `///` is a doc comment after all, and to only omit comments with four slashes or more, which is what this commit implements. Can't really draw boxes that narrow, anyway.