Repository: optuna/optuna

Issue:
Unify the documentation of visualisation module
<!-- Please write a clear and concise description of what content in https://optuna.readthedocs.io/ is an issue. -->

This is a ​part of #2959. A few pages of `optuna.visualization.matplotlib.*` is less informative than plotly's one. Even though all matplotlib's page refer to reading plotly's page, we can give more user-friendly pages. In my best knowledge, the following part can be addressed at least.

- [ ] `matplotlib.plot_edf`
- [ ] `matplotlib.plot_parallel_coordinate`

---


##  `optuna.visualization.matplotlib.plot_edf`

### Why?
[`optuna.visualization.plot_edf`](https://optuna.readthedocs.io/en/latest/reference/visualization/generated/optuna.visualization.plot_edf.html) says


> Note that only the complete trials are considered when plotting the EDF.

However, [`optuna.visualization.matplotlib.plot_edf`](https://optuna.readthedocs.io/en/latest/reference/visualization/generated/optuna.visualization.matplotlib.plot_edf.html#optuna.visualization.matplotlib.plot_edf) does not mention it.

### How to address it?
Please add the same note section to the docs of `optuna.visualization.matplotlib.plot_edf`.

---

##  `optuna.visualization.matplotlib.plot_parallel_coordinate`

### Why?
[`optuna.visualization.plot_parallel_coordinate`](https://optuna.readthedocs.io/en/latest/reference/visualization/generated/optuna.visualization.plot_parallel_coordinate.html) says


> Note that, if a parameter contains missing values, a trial with missing values is not plotted.

However, [`optuna.visualization.matplotlib.plot_parallel_coordinate`](https://optuna.readthedocs.io/en/latest/reference/visualization/generated/optuna.visualization.matplotlib.plot_edf.html#optuna.visualization.matplotlib.plot_parallel_coordinate) does not mention it.

### How to address it?
Please add the same note section to the docs of `optuna.visualization.matplotlib.plot_parallel_coordinate`.



Discussion:

Resolving patch:
diff --git a/optuna/visualization/matplotlib/_edf.py b/optuna/visualization/matplotlib/_edf.py
--- a/optuna/visualization/matplotlib/_edf.py
+++ b/optuna/visualization/matplotlib/_edf.py
@@ -33,6 +33,8 @@ def plot_edf(
 ) -> "Axes":
     """Plot the objective value EDF (empirical distribution function) of a study with Matplotlib.
 
+    Note that only the complete trials are considered when plotting the EDF.
+
     .. seealso::
         Please refer to :func:`optuna.visualization.plot_edf` for an example,
         where this function can be replaced with it.
diff --git a/optuna/visualization/matplotlib/_parallel_coordinate.py b/optuna/visualization/matplotlib/_parallel_coordinate.py
--- a/optuna/visualization/matplotlib/_parallel_coordinate.py
+++ b/optuna/visualization/matplotlib/_parallel_coordinate.py
@@ -39,6 +39,8 @@ def plot_parallel_coordinate(
 ) -> "Axes":
     """Plot the high-dimensional parameter relationships in a study with Matplotlib.
 
+    Note that, if a parameter contains missing values, a trial with missing values is not plotted.
+
     .. seealso::
         Please refer to :func:`optuna.visualization.plot_parallel_coordinate` for an example.
 


Test patch: