Make typeChecker param explicitly TypeChecker | undefined

Author: sjbarag

src/services/types.ts

@@ -32,7 +32,7 @@ namespace ts {
         getEscapedName(): __String;
         getName(): string;
         getDeclarations(): Declaration[] | undefined;
-        getDocumentationComment(typeChecker?: TypeChecker): SymbolDisplayPart[];
+        getDocumentationComment(typeChecker: TypeChecker | undefined): SymbolDisplayPart[];
         getJsDocTags(): JSDocTagInfo[];
     }
 
@@ -55,7 +55,7 @@ namespace ts {
         getTypeParameters(): TypeParameter[] | undefined;
         getParameters(): Symbol[];
         getReturnType(): Type;
-        getDocumentationComment(typeChecker?: TypeChecker): SymbolDisplayPart[];
+        getDocumentationComment(typeChecker: TypeChecker | undefined): SymbolDisplayPart[];
         getJsDocTags(): JSDocTagInfo[];
     }