Fix context api and document (#727)

* Fix Dial function crash instance when there is no instance context in the ctx

* check ctx to fix. (#841)

* Feat: core.ToContext(ctx, v) for ctx initialization (#852)

* remove exported API: toContext

* Remove unnecessary API

* rework document for API

* fix: make sure the ctx is propagated to connections by detached connection

Co-authored-by: Shelikhoo <xiaokangwang@outlook.com>
Co-authored-by: rurirei <72071920+rurirei@users.noreply.github.com>