Using `@ref` with non-existent multiple-word header names

[barucden]

Consider the following markdown documentation:

[Two words](@ref).

[Different words](@ref).

## Two words

The first link is correctly interpreted as a reference to the header "Two words".

The second link is more problematic. In the process of resolving the link, Documenter creates the slug Different-words for it, and since there is no header named "Different words", Documenter will treat Different-words to be a link to a docstring Different - words (i.e., invocation of Base.:(-)). In most code bases, the user gets a warning

┌ Warning: Cannot resolve @ref for md"[Different words](@ref)" in src/index.md.
│ - No docstring found in doc for binding `Base.-`.
└ @ Documenter ~/julia/Documenter.jl/src/utilities/utilities.jl:49

However, in julia's codebase, there is a docstring for Base.:(-) and so [Different words](@ref) would be resolved as a link to that function's docstring. This behavior is not optimal because it falsely succeeds in resolving non-existent links (see JuliaLang/julia#58073).

A quick-and-dirty solution would be to add the following at the beginning of find_docref:

# Check if the slug matches multiple dash-delimited words
if occursin(r"^\w+(-\w+){1,}$", code)
    return (
        error = "unable to get the binding for `$code` in module $(mod)",
        exception = nothing,
    )
end

[Rahban1]

I checked through the cross_references.jl and it looked fine to me. I think the problem is in HTMLWriter.jl, especially in here. I think we need to add some if checks here, I can open a PR to fix it once @mortenpi confirms that I am heading in the right direction.

[barucden]

I think the problem is in cross_references.jl. I proposed a work-in-progress solution at #2678.