build(docs): fix description comments sometimes empty

Follow-up commit for 4879dc4.

---

Disables concatenation of multiple leading comments for a TypeScript node. Since all shipped source files have a license banner at top, the license banner comment would be incorrectly considered as "comment" for the first TypeScript node of a given file. Since there are various files in the Material project where the first node of a source file is exported and should only use the first leading comment, we need to disable comment concatenation.

See for example: https://github.com/angular/material2/blob/master/src/cdk/coercion/boolean-property.ts

Author: devversion