Mention case where host does not set state context
This should simply fall back to the project context.
///
/// The host is responsible to declare the context the state operation in which it is happening.
///
-/// @note if an unknown context index is provided, it should be handled as
-/// `CLAP_STATE_CONTEXT_PROJECT`.
-///
+/// @note If an unknown context type is provided or if the host does not call
+/// clap_plugin_state_context->set() at all, then the state context type should
+/// be treated as `CLAP_STATE_CONTEXT_PROJECT`.
#ifdef __cplusplus
extern "C" {
