Hoist rich directory rendering, add note on benchmark defaults

This is to make it a little clearer what the lookup process for a fixture looks like.

docs/cli/fixtures.md

@@ -10,7 +10,7 @@ Instead, nnbench borrows a bit of pytest's fixture concept to source parameters
 Suppose you have a benchmark defined in a single file, `metrics.py`:
 
 ```python
-# benchmarks.py
+# metrics.py
 import nnbench
 
 
@@ -20,15 +20,18 @@ def accuracy(model, data):
 ```
 
 To supply `model` and `data` to the benchmark, define both values as return values of similarly named functions in a `conf.py` file in the same directory.
+The layout of your benchmark directory should look like this:
 
-```python
-# Benchmark directory layout:
-# 📂 benchmarks
-# ┣━━ conf.py
-# ┣━━ metrics.py
-# ┣━━ ...
-# ---------------------------------------------------
+```commandline
+📂 benchmarks
+┣━━ conf.py
+┣━━ metrics.py
+┣━━ ...
+```
 
+Inside your `conf.py` file, you might define your values as shown below. Note that currently, all fixtures must be raw Python callables, and must match input values of benchmarks exactly.
+
+```python
 # benchmarks/conf.py
 def model():
     return MyModel()
@@ -44,6 +47,10 @@ Then, nnbench will discover and auto-use these values when running this benchmar
 $ nnbench run benchmarks.py 
 ```
 
+!!! Warning
+    Benchmarks with default values for their arguments will unconditionally use those defaults over potential fixtures.
+    That is, for a benchmark `def add(a: int, b: int = 1)`, only the named parameter `a` will be resolved.
+
 ## Fixtures with inputs
 
 Like in pytest, fixtures can consume inputs. However, in nnbench, fixtures can consume other inputs by name only within the same module scope, i.e. members within the same `conf.py`.