Name the two section rules apart in API.md #234

Problem

apps/hocuspocus.server/API.md uses "section" for two rules.

• The changes route and the MCP section body use one rule: a heading plus the top-level nodes up to the next heading of any level (§The section tree, and §MCP "Section rules"). CONTEXT.md already names this rule Section.
• The caller-side rule and the get_outline tree use another: a heading holds every later node up to the next heading of the same or a smaller level ("A section is a caller-side rule", and the get_outline row).

Readers mistook the second meaning for a defect twice.

What to do

Give the caller-side rule its own name, for example Heading subtree. Use it in both places in API.md, and add it to CONTEXT.md next to Section. Documentation only.

Acceptance

• API.md uses "section" only for the any-level rule.
• The caller-side rule and the get_outline row use the new name.
• CONTEXT.md defines the new name and says how it differs from Section.

Notes

Related to #163. This issue does not wait on it.