Document rsvg-convert requirement for building PDF docs

Author: Mustaphayinka

documentation/start-documenting.rst

@@ -207,8 +207,16 @@ To list other supported :program:`make` targets, run:
 
       .\make help
 
+      
 See :cpy-file:`Doc/README.rst` for more information.
 
+.. note::
+
+   Some targets, such as ``dist-pdf``, require additional system
+   dependencies beyond what's installed in the venv. Building the
+   PDF documentation requires the ``rsvg-convert`` utility to be
+   available on your system.
+
 
 .. _using-sphinx-build:
 .. _doc-build-sphinx:
@@ -239,4 +247,4 @@ replace ``html`` above with the desired builder ``name``.
 .. _Sphinx builder: https://www.sphinx-doc.org/en/master/usage/builders/index.html
 .. _Sphinx Lint: https://github.com/sphinx-contrib/sphinx-lint
 .. _venv-activate: https://packaging.python.org/en/latest/guides/installing-using-pip-and-virtual-environments/#activating-a-virtual-environment
-.. _venv-create: https://packaging.python.org/en/latest/guides/installing-using-pip-and-virtual-environments/#creating-a-virtual-environment
+.. _venv-create: https://packaging.python.org/en/latest/guides/installing-using-pip-and-virtual-environments/#creating-a-virtual-environment