Allow lines to start with `@foo` inside a block tag like `{@code}`

[cpovirk]

IIRC, we currently consider such an appearance of @foo to be an invalid tag (or, if it is a recognized one like @return, maybe a valid tag? or maybe never a valid tag because we're upset that the previous inline tag was never closed?). It is invalid... until Java 15, at which point https://bugs.openjdk.org/browse/JDK-8241780 saves us.

On those grounds, we don't format the Javadoc at all (which is not necessarily a great failure mode, but that's another issue).

Maybe we should just allow this, at least for cases that are not normally Javadoc tags, like @Override?