The ownership comment in `jv.h` currently says every `jv_*` function
except `jv_copy` consumes its input. Several accessors and
object-iteration functions borrow their arguments instead, which makes
that guidance misleading for callers and language bindings.
Document the borrowing exceptions beside their declarations, including
numeric helpers and `jv_show`. Clarify that returned `jv` values are
owned, while the pointers returned by `jv_string_value` and
`jv_number_get_literal` belong to their input values. This changes
comments only.
