build: fix docs-content missing some inherited member description

[devversion]

This happened due to: 9973f1d

Recently we added logic to exclude resolved external API docs from the Dgeni
pipeline, preventing them from being processed by the Dgeni JSDoc processor.

This was necessary due to a bug in Dgeni which resulted in runtime errors for
unknown JSDoc tags. Some tags from external API docs were unknown because
the framework repository uses different sets of annotations/tags.

At this point, it seemed reasonable to never add such external API docs to the
Dgeni pipeline, while also working around this runtime exception that way.

Now after seeing the results, it became clear that we still want to continue
processing external API docs as some of our public-facing API docs might end
up merging external API document members.

In this docs-content built it can be observed that some inherited/merged
members no longer have a proper resolved description:

angular/material2-docs-content@6b602cf

This commit fixes it by adding these external JSDoc tags temporarily as known tags until
the Dgeni upstream fix is available. As part of this change we introduced a custom processor
to error for unknown JSDoc tags (we want to be strict about this). This will become helpful
as Dgeni only warns about unknown tags (and this is non-configurable). Also while being at it
the types for tags has been improved to avoid the use of any everywhere.

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