docs: ignore FixtureFunctionDefinition from docs

This is an internal class users don't need to know about it.

Author: Glyphack

Diff for: doc/en/conf.py

@@ -185,6 +185,7 @@
     ("py:class", "_pytest._code.code.TerminalRepr"),
     ("py:class", "TerminalRepr"),
     ("py:class", "_pytest.fixtures.FixtureFunctionMarker"),
+    ("py:class", "_pytest.fixtures.FixtureFunctionDefinition"),
     ("py:class", "_pytest.logging.LogCaptureHandler"),
     ("py:class", "_pytest.mark.structures.ParameterSet"),
     # Intentionally undocumented/private