docs: clarify that feature directories may be outside the project root

[mnriem]

Problem

docs/reference/core.md describes SPECIFY_FEATURE_DIRECTORY as selecting a feature directory “within” the resolved project (environment-variable table and two-axes note). docs/guides/monorepo.md likewise says the selected feature is “within” the project. This implies a containment rule that is not intended or enforced.

Feature directories hold documentation artifacts such as spec.md and plan.md and may intentionally be outside the project root. The project root identifies the Spec Kit project and its command/configuration context; choosing an external feature directory does not relocate or select executable skills, scripts, or extensions.

Requested clarification

• Update both pages to distinguish the project root from the feature-document location.
• Preserve the current explanation that relative feature paths resolve against the selected project root, while making clear that an absolute path (or a relative path resolving outside the root) may select an external feature directory.
• Clarify that the default feature path is under specs/, but explicitly selected feature paths are not restricted to the project root.

This is documentation-only: do not add path containment or change runtime behavior.