better error for help on invalid characters

[inkydragon]

Set Difference: \setminus, it should equal to setdiff()

error can be reproducted on: v0.64/v0.7/v1.0/v1.02/v1.0.3

help?> ∖
ERROR: LoadError: Invalid @var syntax `invalid character "∖"`.
Stacktrace:
 [1] error(::String) at .\error.jl:33
 [2] splitexpr(::Any) at .\docs\bindings.jl:28
 [3] bindingexpr(::Any) at .\docs\bindings.jl:17
 [4] lookup_doc(::Expr) at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\REPL\src\docview.jl:137
 [5] docm(::LineNumberNode, ::Module, ::Any) at .\docs\Docs.jl:501
 [6] @doc(::LineNumberNode, ::Module, ::Expr, ::Vararg{Expr,N} where N) at .\boot.jl:451
in expression starting at C:\cygwin\home\Administrator\buildbot\worker\package_win64\build\usr\share\julia\stdlib\v1.0\REPL\src\docview.jl:301

julia> Char('∖')
'∖': Unicode U+2216 (category Sm: Symbol, math)

[JeffBezanson]

Not specific to help mode; we have this marked as an invalid character probably because it is so similar to backslash. It would be possible to parse it as an operator though.

[inkydragon]

So, should we add it to the base just like other set opreater in #6582 ?
or keep the status quo

[Keno]

Seems like help mode could be more helpful here and say that the character is disallowed independent of whether we end up allowing it.

[stevengj]

I would vote in favor of not allowing it, as it is too similar to \.

A help message rather than an exception for non-parseable strings sounds good.