build(aio): split the description property in API docs (#22401)

* The first paragraph is now split off into the `shortDescription` property.
* Usage of `howToUse` and `whatItDoes` have been updated.
* The "Overview" heading for class is removed as it is self-evident
* The original horizontal rule styling below the main heading is removed as not part of the new design

Closes #22385

PR Close #22401

aio/tools/transforms/angular-api-package/processors/splitDescription.spec.js

@@ -0,0 +1,71 @@
+const testPackage = require('../../helpers/test-package');
+const processorFactory = require('./splitDescription');
+const Dgeni = require('dgeni');
+
+describe('splitDescription processor', () => {
+
+  it('should be available on the injector', () => {
+    const dgeni = new Dgeni([testPackage('angular-api-package')]);
+    const injector = dgeni.configureInjector();
+    const processor = injector.get('splitDescription');
+    expect(processor.$process).toBeDefined();
+  });
+
+  it('should run before the correct processor', () => {
+    const processor = processorFactory();
+    expect(processor.$runBefore).toEqual(['processing-docs']);
+  });
+
+  it('should run after the correct processor', () => {
+    const processor = processorFactory();
+    expect(processor.$runAfter).toEqual(['tags-extracted', 'migrateLegacyJSDocTags']);
+  });
+
+  it('should split the `description` property into the first paragraph and other paragraphs', () => {
+    const processor = processorFactory();
+    processor.docTypes = ['test'];
+    const docs = [
+      { docType: 'test' },
+      { docType: 'test', description: '' },
+      { docType: 'test', description: 'abc' },
+      { docType: 'test', description: 'abc\n' },
+      { docType: 'test', description: 'abc\n\n' },
+      { docType: 'test', description: 'abc\ncde' },
+      { docType: 'test', description: 'abc\ncde\n' },
+      { docType: 'test', description: 'abc\n\ncde' },
+      { docType: 'test', description: 'abc\n  \ncde' },
+      { docType: 'test', description: 'abc\n\n\ncde' },
+      { docType: 'test', description: 'abc\n\ncde\nfgh' },
+      { docType: 'test', description: 'abc\n\ncde\n\nfgh' },
+    ];
+    processor.$process(docs);
+    expect(docs).toEqual([
+      { docType: 'test' },
+      { docType: 'test', shortDescription: '', description: '' },
+      { docType: 'test', shortDescription: 'abc', description: '' },
+      { docType: 'test', shortDescription: 'abc', description: '' },
+      { docType: 'test', shortDescription: 'abc', description: '' },
+      { docType: 'test', shortDescription: 'abc\ncde', description: '' },
+      { docType: 'test', shortDescription: 'abc\ncde', description: '' },
+      { docType: 'test', shortDescription: 'abc', description: 'cde' },
+      { docType: 'test', shortDescription: 'abc', description: 'cde' },
+      { docType: 'test', shortDescription: 'abc', description: 'cde' },
+      { docType: 'test', shortDescription: 'abc', description: 'cde\nfgh' },
+      { docType: 'test', shortDescription: 'abc', description: 'cde\n\nfgh' },
+    ]);
+  });
+
+  it('should ignore docs that do not match the specified doc types', () => {
+    const processor = processorFactory();
+    processor.docTypes = ['test'];
+    const docs = [
+      { docType: 'test', description: 'abc\n\ncde' },
+      { docType: 'other', description: 'abc\n\ncde' }
+    ];
+    processor.$process(docs);
+    expect(docs).toEqual([
+      { docType: 'test', shortDescription: 'abc', description: 'cde' },
+      { docType: 'other', description: 'abc\n\ncde' }
+    ]);
+  });
+});