Bug summary

Usign Latex fonts as recommended here throws a Warning:

findfont: Font family ['serif'] not found. Falling back to DejaVu Sans.
findfont: Generic family 'serif' not found because none of the following families were found: 

Code for reproduction

import matplotlib.pyplot as plt

plt.rcParams.update({
    "font.family": "serif",
    # Use LaTeX default serif font.
    "font.serif": [],
    # Use specific cursive fonts.
    "font.cursive": ["Comic Neue", "Comic Sans MS"],
})

plt.figure()
plt.plot([1,2,3,4])
plt.savefig('test.png')
plt.savefig('test.pgf')

Actual outcome

The Bug is also visible in your Docs.
https://matplotlib.org/stable/gallery/userdemo/pgf_fonts.html

Expected outcome

No Warning should get thrown everytime

Operating system

Ubuntu

Matplotlib Version

3.4.2

Matplotlib Backend

TkAgg

Python version

3.9.5

Jupyter version

No response

Other libraries

No response

Installation

pip

Conda channel

No response