[docs] FixtureManager documentation not included in reference

[svenevs]

I wanted to link to _pytest.FixtureManager.getfixtureclosure in my documentation and noticed that this class is not included in the reference.rst. I would ordinarily submit a pull request, but

1. I wasn't sure if this is exactly where the documentation should go.
2. I cannot get class signatures to compile. I had to comment out the usage of 'sphinxcontrib_trio' in extensions of conf.py, otherwise I kept getting errors from that extension. But without it no class signatures are rendered locally.

Anyway, I just stuck it at the end of the Fixtures section, the following diff should achieve the desired result but since I cannot get the docs to compile locally, I'm not sure :/

--- a/doc/en/reference.rst
+++ b/doc/en/reference.rst
@@ -496,6 +496,14 @@ tmpdir_factory
 .. automethod:: TempdirFactory.mktemp
 .. automethod:: TempdirFactory.getbasetemp
 
+FixtureManager
+~~~~~~~~~~~~~~
+
+.. currentmodule:: _pytest.fixtures
+
+.. autoclass:: _pytest.fixtures.FixtureManager
+    :members:
+
 
 .. _`hook-reference`:

[pytestbot]

GitMate.io thinks possibly related issues are #1713 (No reference documentation?), #1068 (Wrong documentation), #795 (Docs typo), #1696 (404 in documentation), and #2076 (Case Scenario long integration test - including reference data).

[nicoddemus]

@svenevs thanks, but feel free to submit a PR anyway, that's why we have CI. 😉

[nicoddemus]

As mentioned in #3430 (comment), the FixtureManager is an internal API so it should not go into the docs. Completely my fault for not realizing this sooner, sorry again for wasting your time @svenevs!