docgen: Omit unknown type tag from Markdown format output

[aduth]

Related: #18045
Unblocks: #19520

This pull request seeks to resolve an issue where, if docgen encounters a type that it cannot parse, the Markdown formatter will output a null literal value for the type. With these changes, it will simply not output a type at all output text indicating that the type is unknown, in order to avoid misleading the reader into thinking the type is null when it is not.

This should be considered an interim solution to the larger problem of substituting Doctrine (#18045) in order to use more complex TypeScript types (#18838).

Testing Instructions:

Verify unit tests pass:

npm run test-unit

Verify there are no changed documentation after running:

npm run docs:build

[gziolo]

It looks like a reasonable way to handle such cases for the time being 👍

[aduth]

I somehow missed that types are output for a number of tags, so I updated in e9da943 to output the "unknown type" more consistently (caught a few more cases in the subsequent 4e10bce).