Support @DeprecationSummary in doc comments

Adds support for authoring `@DeprecationSummary` messages in
documentation comments. This is useful when you want to provide a
formatted documentation summary for an API that's deprecated, but don't
want to create a documentation extension file in order to do just that.

It turns out that DocC already supported this, but emitted a warning
whenever you did so. This commit removes that warning.

Author: franklinsch

Sources/SwiftDocC/Model/DocumentationNode.swift

@@ -849,6 +849,7 @@ public struct DocumentationNode {
 
 private let directivesSupportedInDocumentationComments = [
         Metadata.directiveName,
+        DeprecationSummary.directiveName,
     ]
     // Renderable directives are processed like any other piece of structured markdown (tables, lists, etc.)
     // and so are inherently supported in doc comments.

Sources/SwiftDocC/Semantics/Symbol/DeprecationSummary.swift

@@ -24,7 +24,9 @@ import Markdown
 /// }
 /// ```
 ///
-/// You can use the `@DeprecationSummary` directive top-level in both articles and documentation extension files.
+/// You can use the `@DeprecationSummary` directive top-level in articles, documentation extension files, or documentation comments.
+///
+/// > Earlier versions: Before Swift-DocC 6.1, `@DeprecationSummary` was not supported in documentation comments.
 ///
 /// > Tip:
 /// > If you are writing a custom deprecation summary message for an API or documentation page that isn't already deprecated,

Sources/docc/DocCDocumentation.docc/DocC.symbols.json

@@ -2033,7 +2033,13 @@
             "text" : ""
           },
           {
-            "text" : "You can use the `@DeprecationSummary` directive top-level in both articles and documentation extension files."
+            "text" : "You can use the `@DeprecationSummary` directive top-level in articles, documentation extension files, or documentation comments."
+          },
+          {
+            "text" : ""
+          },
+          {
+            "text" : "> Earlier versions: Before Swift-DocC 6.1, `@DeprecationSummary` was not supported in documentation comments."
           },
           {
             "text" : ""

Tests/SwiftDocCTests/Semantics/SymbolTests.swift

@@ -1313,6 +1313,32 @@ class SymbolTests: XCTestCase {
         )
     }
 
+    func testParsesDeprecationSummaryDirectiveFromDocComment() throws {
+        let (node, problems) = try makeDocumentationNodeForSymbol(
+            docComment: """
+                The symbol's abstract.
+
+                @DeprecationSummary {
+                  This is the deprecation summary.
+                }
+                """,
+            articleContent: nil
+        )
+        
+        XCTAssert(problems.isEmpty)
+        
+        XCTAssertEqual(
+            (node.semantic as? Symbol)?
+                .deprecatedSummary?
+                .content
+                .first?
+                .format()
+                .trimmingCharacters(in: .whitespaces)
+            ,
+            "This is the deprecation summary."
+        )
+    }
+
     // MARK: - Helpers
 
     func makeDocumentationNodeForSymbol(