message: build(aio): implement rules to prevent headings in content
  * No headings are allowed in `description` and `shortDescription`
  * Only heading level 3 is allowed in `usageNotes`
  PR Close
diff: @@ -119,6 +119,24 @@ module.exports = new Package('angular-api', [basePackage, typeScriptPackage])
  addNotYetDocumentedProperty.docTypes = API_DOC_TYPES;
  })
  + .config(function(checkContentRules, EXPORT_DOC_TYPES) {
  + const createNoMarkdownHeadings = require('./content-rules/noMarkdownHeadings');
  + const noMarkdownHeadings = createNoMarkdownHeadings();
  + const allowOnlyLevel3Headings = createNoMarkdownHeadings(1, 2, '4,');
  + const DOC_TYPES_TO_CHECK = EXPORT_DOC_TYPES.concat(['member', 'overload-info']);
  + const PROPS_TO_CHECK = ['description', 'shortDescription'];
  +
  + DOC_TYPES_TO_CHECK.forEach(docType => {
  + const ruleSet = checkContentRules.docTypeRules[docType] = checkContentRules.docTypeRules[docType] || {};
  + PROPS_TO_CHECK.forEach(prop => {
  + const rules = ruleSet[prop] = ruleSet[prop] || [];
  + rules.push(noMarkdownHeadings);
  + });
  + const rules = ruleSet['usageNotes'] = ruleSet['usageNotes'] || [];
  + rules.push(allowOnlyLevel3Headings);
  + });
  + })
  +
  .config(function(computePathsProcessor, EXPORT_DOC_TYPES, generateApiListDoc) {
  const API_SEGMENT = 'api';
  