docs: say what --show-builtin-config resolves

docs: say what --show-builtin-config resolves

The description said the command writes "the built-in configuration for
VERSION", which reads as the contents of the schema file with that name.
It is not: a schema states only the keys it changes, so the command
flattens the whole inheritance chain and prints what the version resolves
to.

The distinction is the one a reader is most likely to get wrong, because
the surprising half looks like a bug -- asking for version 1 lists
options version 1 has never heard of.  Say why that is right (a daemon
pinned to that version resolves them too), and say what happens where the
versions disagree (the selected version's value wins), with the concrete
case.

Assisted-by: Claude-Code:claude-opus-5