fix(documentation): correctly show T? type (#380)

Fixes #375

Author: i582

server/src/e2e/suite/testcases/documentation/functions.test

@@ -181,3 +181,12 @@ Some interesting list
 - Point 1
   with other line 
 - Point 2
+
+========================================================================
+Function documentation with T? type
+========================================================================
+fun <caret>foo(): Int? {}
+------------------------------------------------------------------------
+```tact
+fun foo(): Int?
+```

server/src/psi/Decls.ts

@@ -337,7 +337,9 @@ export class Fun extends NamedNode {
         if (!parametersNode) return ""
 
         const result = this.returnType()
-        return parametersNode.text + (result ? `: ${result.node.text}` : "")
+        const suffix = result?.node.nextSibling?.text === "?" ? "?" : ""
+
+        return parametersNode.text + (result ? `: ${result.node.text}${suffix}` : "")
     }
 
     public isOverride(): boolean {