[PR #12498/b864e501 backport][8.2.x] 🎨 Add descriptions to all tox environments

[patchback]

This is a backport of PR #12498 as merged into main (b864e50).

Previously, a part of the environments weren't documented in the config, making it difficult for the newbies to figure out what their purposes are. This patch sets the descriptions for all the envs listed with the tox -av command, leveraging the dynamic factor-dependent explanation fragments.

default environments:
linting                   -> run pre-commit-defined linters under `python3`
py38                      -> run the tests under `py38`
py39                      -> run the tests under `py39`
py310                     -> run the tests under `py310`
py311                     -> run the tests under `py311`
py312                     -> run the tests under `py312`
py313                     -> run the tests under `py313`
pypy3                     -> run the tests under `pypy3`
py38-pexpect              -> run the tests against `pexpect` under `py38`
py38-xdist                -> run the tests with pytest in parallel mode under `py38`
py38-unittestextras       -> run the tests against the unit test extras under `py38`
py38-numpy                -> run the tests against `numpy` under `py38`
py38-pluggymain           -> run the tests against the bleeding edge `pluggy` from Git under `py38`
py38-pylib                -> run the tests against `py` lib under `py38`
doctesting                -> run the tests under `~/.pyenv/versions/3.12.3/envs/pytest-pyenv-py3.12.3/bin/python` including doctests
doctesting-coverage       -> run the tests collecting coverage under `~/.pyenv/versions/3.12.3/envs/pytest-pyenv-py3.12.3/bin/python` including doctests
plugins                   -> run reverse dependency testing against pytest plugins under `~/.pyenv/versions/3.12.3/envs/pytest-pyenv-py3.12.3/bin/python`
py38-freeze               -> test pytest frozen with `pyinstaller` under `py38`
docs                      -> build the documentation site under `~/src/github/pytest-dev/pytest/doc/en/_build/html` with `python3`
docs-checklinks           -> check the links in the documentation with `python3`
py311-exceptiongroup      -> run the tests against `exceptiongroup` under `py311`

additional environments:
regen                     -> regenerate documentation examples under `python3`
release                   -> do a release, required posarg of the version number
prepare-release-pr        -> prepare a release PR from a manual trigger in GitHub actions
generate-gh-release-notes -> generate release notes that can be published as GitHub Release
nobyte                    -> run the tests in no-bytecode mode under `~/.pyenv/versions/3.12.3/envs/pytest-pyenv-py3.12.3/bin/python`
lsof                      -> run the tests with `--lsof` pytest CLI option under `~/.pyenv/versions/3.12.3/envs/pytest-pyenv-py3.12.3/bin/python`