docs: explain drawing hidden managed references

[mackysoft]

Summary

An explicitly drawn [HideInInspector] managed reference can show its selector without its subtype fields. Document how a custom editor can display those fields using Unity's existing SerializedObject.forceChildVisibility API.

Scope

• Add a README FAQ with a drawing scope that restores the previous visibility setting in finally.
• Explain that the setting also reveals individually hidden descendants, and that screen-specific filtering belongs in the custom editor when those hiding rules should be preserved.
• This is usage guidance; automatic drawer visibility behavior is unchanged.

Verification

• Reproduced the reported omission with package 1.6.1 on Unity 2022.3.62f3, both in a standalone EditorWindow and an xNode GlobalNodeEditor subclass. Standard Unity PropertyField without SubclassSelector showed the same omission.
• Seven isolated EditMode GUI cases passed against main 9587592: child rendering, keyboard editing and application, restoration of the caller's visibility setting, a separate SerializedObject retaining its default setting, and ordinary Inspector hiding.
• git diff --check passed. The repository change is documentation only.
• The same commit passed the Tests workflow, including Verification; the EditMode matrix was skipped by the documentation-only change filter.

Closes #85

[mackysoft]

Closing this PR because the investigation result and the existing Unity API workaround are sufficiently covered by the reply to #85. A README addition is unnecessary for this issue.