Clarify heading marker rename semantics in patch docs

Clients renaming a heading via targetScope 'marker'/'markerAndContent' were
passing bare heading text instead of the full heading line, silently
stripping the '#' characters and demoting the heading to a plain paragraph.
The vault_patch tool description and REST targeting docs now explain that
the marker is the literal heading line including its '#' characters, and
that depth must be inferred from the number of '::'-delimited segments in
the target path since vault_get_document_map only returns heading paths,
not raw markdown. Stripping the '#' characters is still documented as a
valid way to intentionally demote a heading to plain text.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>