Search in current document subtree by default

Users will most likely use this macro to search in the current document subtree and expect the macro to behave like this by default.

A solution to achieve this by default is to set up the reference parameter to have "WebHome" as its default parameter, and still allow it to be set to "" to search everywhere.

Technically this is a breaking change, but the macro is currently not used much so this will likely not break anybody, especially that the reference has been allowed to be empty of a very short time.