The description of the heading option does not mention, that this option only works when also separate_signatureis set to true.

Relevant code snippets

Compare jinja code (here only for modules, equally for other types):

{% if config.separate_signature %}
  <span class="doc doc-object-name doc-module-name">{{ config.heading if config.heading and root else module_name }}</span>
{% else %}
  <code>{{ module_name }}</code>
{% endif %}

Documentation Sections:

• headings
• signatures
• Original change in issue feature: Allow custom headings for identifiers #725

Further comments

I filed this as a documentation bug, since this is the easiest to resolve, however I'd also consider this a bug/feature request, to have the heading option working even without separate_signature option. On request I can add another issue for that.