Add possibility to include private symbols.

Changelog.md

@@ -7,6 +7,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+
+- Added support for generating documentation for
+  private symbols.
+  #266 by @Lukas-Stuehrk
+
 ### Fixed
 
 - Fixed bug that caused prefix and postfix operators to be omitted

Sources/swift-doc/Supporting Types/AccessLevel.swift

@@ -4,13 +4,16 @@ import SwiftDoc
 enum AccessLevel: String, ExpressibleByArgument {
     case `public`
     case `internal`
+    case `private`
 
     func includes(symbol: Symbol) -> Bool {
         switch self {
         case .public:
             return symbol.isPublic
         case .internal:
             return symbol.isPublic || symbol.isInternal
+        case .private:
+            return true
         }
     }
 }