@_nodoc attribute for hiding symbols from the symbol graph

[xwu (Xiaodi Wu)]

@taylorswift

i don’t feel strongly about this, but from an educational perspective, it would feel weird to explain why the @documentation(visibility: public) is needed here:

// isn’t ``_ChannelInboundHandler`` already public?
@documentation(visibility: public)
public 
protocol _ChannelInboundHandler 

How about...let's just make it not needed here.

With an explicit attribute to toggle visibility plus command line options for symbol graph generation plus filtering on the DocC side, there is no reason that a convention about underscored APIs should continue to be hardcoded as yet another standalone feature rather than a default that's overridable at the command line and/or filterable via DocC.