rustdoc: make calls of markdown::render safer

rustdoc: make calls of markdown::render safer

Previously, external code might call `markdown::render()` without having
called `markdown::reset_headers()`, meaning the TLS key
`used_header_map` was unset.  Now `markdown::render()` ensures that
`used_header_map` is set by calling `reset_headers` if necessary.

Fix #17736