Doc: add explainer for Escapable

resolves rdar://146331729

Author: kavon

stdlib/public/core/Misc.swift

@@ -239,7 +239,20 @@ func _rethrowsViaClosure(_ fn: () throws -> ()) rethrows {
 /// Extensions to the `Copyable` protocol are not allowed.
 @_marker public protocol Copyable/*: ~Escapable*/ {}
 
-@_documentation(visibility: internal)
+/// A protocol representing types whose values can persist beyond the immediate local context.
+///
+/// They may be freely assigned to global or static variables, returned from functions, or captured by escaping
+/// closures, etc. All Swift types implicitly conform to this protocol by default, allowing them to be moved across
+/// scopes freely because they lack any lifetime dependencies.
+///
+/// In contrast, values of types that suppress their implicit conformance to `Escapable` (by writing `~Escapable`)
+/// carry a lifetime dependency. These dependencies ensure the `~Escapable` value does not live longer than the value it
+/// depends on. Explicit lifetime dependency annotations may be required when working with these types.
+///
+/// In generic contexts, `~Escapable` works much in the same way as `~Copyable`. It allows functions and types to work
+/// with values that may or may not be Escapable, and types can be conditionally Escapable based on their generic
+/// arguments. A conformance requirement for Escapable is automatically inferred in extensions and for generic type
+/// parameters, unless suppressed with `~Escapable`.
 @_marker public protocol Escapable/*: ~Copyable*/ {}
 
 @_marker public protocol BitwiseCopyable: ~Escapable { }