docs: Clarify difference between PYTEST_PLUGINS and -p option

Added a note to the PYTEST_PLUGINS environment variable documentation explaining the difference between PYTEST_PLUGINS and the -p command-line option. This addresses issue #13388 by clarifying that PYTEST_PLUGINS is processed before command-line arguments, making it suitable for always loading plugins, while -p is better for one-time plugin loading or testing.

Author: shiva-39