It is referring to the registerProtocolHandler function. The function allows you to register handlers for custom schemes, so that you can do clever things with links like <a href="awesome://example.com">awesome</a>.
The "web+something" namespace is what we should be using for our custom scheme handlers, but there are a bunch of other web-related schemes that will be fine to use too... "bitcoin" is one of the OK ones.
It's not a big deal as far as I can see... basically any scheme that is popular enough or documented enough and isn't supposed to launch some external application could probably get white-listed if you asked nicely.
The "web+something" namespace is what we should be using for our custom scheme handlers, but there are a bunch of other web-related schemes that will be fine to use too... "bitcoin" is one of the OK ones.
It's not a big deal as far as I can see... basically any scheme that is popular enough or documented enough and isn't supposed to launch some external application could probably get white-listed if you asked nicely.