Docs: C annotations extension produces invalid docutils tree

[m-aciek]

The function add_annotations in Doc/tools/extensions/c_annotations.py inserts an emphasis node into a desc_content node. This produces an invalid document tree, since the desc_content node is a docutils Element and is not supposed to have children that are text or inline elements. To fix this, the emphasis node needs to be wrapped in a paragraph node.

[…] the validity of the docutils tree is not checked by docutils or Sphinx.

This prevents to build the documentation to PDF with rinohtype.