Fix for @Option(transform:) with optional type (#619)

Due to the restructuring in #477, there was ambiguity between the
unconstrained `@Option` initializer that uses a transform (but no
initial value) and the one that is constrained to the property being
optional. This marks the unconstrained version as disfavored, which
allows overload resolution to select the optional version when
appropriate.

Also fixes this for `@Argument` and improves documentation
consistency for `@Option`.

Fixes #618.

Sources/ArgumentParser/Documentation.docc/Extensions/Option.md

@@ -2,15 +2,15 @@
 
 ## Topics
 
-### Single Options
+### Single-Value Options
 
-- ``init(name:parsing:help:completion:)-7slrf``
 - ``init(name:parsing:help:completion:)-4yske``
+- ``init(name:parsing:help:completion:)-7slrf``
+- ``init(wrappedValue:name:parsing:help:completion:)-7ilku``
 - ``init(name:parsing:help:completion:transform:)-2wf44``
 - ``init(name:parsing:help:completion:transform:)-25g7b``
 - ``init(wrappedValue:name:parsing:help:completion:transform:)-2llve``
 - ``SingleValueParsingStrategy``
-- ``init(wrappedValue:name:parsing:help:completion:)-7ilku``
 
 ### Array Options
 

Sources/ArgumentParser/Parsable Properties/Argument.swift

@@ -414,6 +414,7 @@ extension Argument {
   ///   - transform: A closure that converts a string into this property's
   ///     element type or throws an error.
   @preconcurrency
+  @_disfavoredOverload
   public init(
     help: ArgumentHelp? = nil,
     completion: CompletionKind? = nil,