client spec: fix page platform section

[sbrl]

This updates the page platform section to match what we actually do, following a discussion with @agnivade via Gitter.

@agnivade, how to you mean that the Page Resolution section needs clarification?

[agnivade]

We can add a line saying that - It is possible that due to this page resolution logic, the client may show a page which does not belong to the host platform, because a page can reside in common, and not in the host platform. So the user must not assume that a shown command is always runnable in the host platform.

There is also this issue that now we do not know exactly which platforms are supported with a command. That would need a page structure change to add metadata to specify this.

[prfcttone2]

Great idea