fix: better handling of union return types

Fixes #93

It's non trivial to support this syntax so let's spit out a helpful error message instead.

src/__tests__/markdown-helpers.spec.ts

@@ -474,6 +474,17 @@ foo`),
       });
     });
 
+    it('should helpfully error for badly formatted union return types', () => {
+      const customTokens = getTokens(
+        `Returns \`WebContents\` | \`string\` - A WebContents instance with the given ID.`,
+      );
+      expect(() => extractReturnType(customTokens)).toThrowErrorMatchingInlineSnapshot(`
+        "Found a return type declaration that appears to be declaring a type union (A | B) but in the incorrect format. Type unions must be fully enclosed in backticks. For instance, instead of \`A\` | \`B\` you should specify \`A | B\`.
+        Specifically this error was encountered here:
+          "Returns \`WebContents\` | \`string\` - A WebContents instance with the given ID."..."
+      `);
+    });
+
     it('should handle return types with no descriptions', () => {
       const printerTokens = getTokens(`Returns [\`PrinterInfo[]\`](structures/printer-info.md)`);
       const printerRet = extractReturnType(printerTokens);

src/markdown-helpers.ts

@@ -530,6 +530,12 @@ export const extractReturnType = (
     } catch {}
   }
 
+  if (parsedDescription.trim().startsWith('|')) {
+    throw new Error(
+      `Found a return type declaration that appears to be declaring a type union (A | B) but in the incorrect format. Type unions must be fully enclosed in backticks. For instance, instead of \`A\` | \`B\` you should specify \`A | B\`.\nSpecifically this error was encountered here:\n  "${rawDescription.trim().slice(0, 100)}"...`,
+    );
+  }
+
   return {
     parsedDescription: parsedDescription.trim(),
     parsedReturnType: rawTypeToTypeInformation(rawReturnType, parsedDescription, typedKeys),