Add qldoc for three public predicates in `PrintAst.qll`.
Add qldoc for three public predicates in `PrintAst.qll`.

It's bland, but we try to maintain a 100% documentation coverage for our public library elements.

github/codeql-go,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql-go,github/codeql,github/codeql-go,github/codeql,github/codeql,github/codeql,github/codeql,github/codeql-go,github/codeql,github/codeql,github/codeql,github/codeql