make sure `_doc` always returns a String

[pfitzseb]

Should fix a

ERROR: LoadError: MethodError: Cannot `convert` an object of type Text{String} to an object of type String
Closest candidates are:
  convert(::Type{String}, !Matched::CategoricalArrays.CategoricalValue) at C:\Users\...\.julia\packages\CategoricalArrays\0ZAbp\src\value.jl:60
  convert(::Type{String}, !Matched::FilePathsBase.AbstractPath) at C:\Users\...\.julia\packages\FilePathsBase\9nTwN\src\path.jl:112
  convert(::Type{String}, !Matched::LibPQ.PQValue) at C:\Users\...\.julia\packages\LibPQ\LSA5x\src\parsing.jl:105
  ...
Stacktrace:
 [1] Main.SymbolServer.FunctionStore(::Main.SymbolServer.VarRef, ::Array{Main.SymbolServer.MethodStore,1}, ::Text{String}, ::Main.SymbolServer.VarRef, ::Bool) at c:\Users\...\.vscode-insiders\extensions\julialang.language-julia-insider-1.0.10\scripts\packages\SymbolServer\src\symbols.jl:76
 [2] Main.SymbolServer.FunctionStore(::Any, ::Module, ::Bool) at c:\Users\...\.vscode-insiders\extensions\julialang.language-julia-insider-1.0.10\scripts\packages\SymbolServer\src\symbols.jl:87
 [3] symbols(::Dict{Symbol,Main.SymbolServer.ModuleStore}, ::Module, ::Base.IdSet{Symbol}, ::Base.IdSet{Module}) at c:\Users\...\.vscode-insiders\extensions\julialang.language-julia-insider-1.0.10\scripts\packages\SymbolServer\src\symbols.jl:402
 [4] symbols(::Dict{Symbol,Main.SymbolServer.ModuleStore}, ::Nothing, ::Base.IdSet{Symbol}, ::Base.IdSet{Module}) at c:\Users\...\.vscode-insiders\extensions\julialang.language-julia-insider-1.0.10\scripts\packages\SymbolServer\src\symbols.jl:433
 [5] top-level scope at c:\Users\...\.vscode-insiders\extensions\julialang.language-julia-insider-1.0.10\scripts\packages\SymbolServer\src\server.jl:148
 [6] include(::Function, ::Module, ::String) at .\Base.jl:380
 [7] include(::Module, ::String) at .\Base.jl:368
 [8] exec_options(::Base.JLOptions) at .\client.jl:296
 [9] _start() at .\client.jl:506

error we got on Azure.

[davidanthoff]

Isn't this more a bug in some package? Text{String} is probably not a type that Base.getdocs should return, right?

We need to harden against it, of course, but I'm wondering whether we should rather just assume that there isn't a doc string in such a case, i.e. return an empty string?

We should probably at some point also think about some way that we somehow record things that look like errors in packages that we discover during indexing, and then report them back to the packages in some way.

[pfitzseb]

The docs say that

The returned object can either be a markdown object generated by Markdown.parse or some other custom type used to display non-markdown formatted documentation.

so this seems to be intentional.