Perhaps i'm leaning too much on language-spec/compiler-internal jargon, but at least there it's established that a symbol being "underscored" means "the name starts with an underscore"; various tools use this to mean "the symbol should be treated as internal to the package", including the symbol graph and Swift-DocC. I chose these instead of my previously-suggested forceVisible and forceHidden since those are also misleading (using forceVisible on an internal symbol won't make it start appearing in public docs, and using forceHidden won't hide the symbol from all docs, just public ones).