Docs: clarify -p vs PYTEST_PLUGINS plugin loading (#14113)

Closes #13388

Author: Dooopinder

changelog/13388.doc.rst

@@ -0,0 +1 @@
+Clarified documentation for ``-p`` vs ``PYTEST_PLUGINS`` plugin loading and fixed an incorrect ``-p`` example.

doc/en/how-to/plugins.rst

@@ -158,7 +158,7 @@ manually specify each plugin with :option:`-p` or :envvar:`PYTEST_PLUGINS`, you
 
 .. code-block:: bash
 
-   pytest --disable-plugin-autoload -p NAME,NAME2
+   pytest --disable-plugin-autoload -p NAME -p NAME2
 
 .. tab:: toml
 
@@ -180,3 +180,31 @@ manually specify each plugin with :option:`-p` or :envvar:`PYTEST_PLUGINS`, you
 .. versionadded:: 8.4
 
    The :option:`--disable-plugin-autoload` command-line flag.
+
+.. note::
+
+   :option:`-p` and :envvar:`PYTEST_PLUGINS` are both ways to explicitly control which
+   plugins are loaded, but they serve slightly different use-cases.
+
+   * :option:`-p` loads (or disables with ``-p no:<name>``) a plugin by name or entry point
+     for a specific pytest invocation, and is processed early during startup.
+   * :envvar:`PYTEST_PLUGINS` is a comma-separated list of Python modules that are imported
+     and registered as plugins during startup. This mechanism is commonly used by test
+     suites, for example when testing a plugin.
+
+   When explicitly controlling plugin loading (especially with
+   :envvar:`PYTEST_DISABLE_PLUGIN_AUTOLOAD` or :option:`--disable-plugin-autoload`),
+   avoid specifying the same plugin via multiple mechanisms. Registering the same plugin
+   more than once can lead to errors during plugin registration.
+
+Examples:
+
+.. code-block:: bash
+
+   # Disable auto-loading and load only specific plugins for this invocation
+   PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 pytest -p xdist
+
+.. code-block:: bash
+
+   # Disable auto-loading and load plugin modules during startup
+   PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 PYTEST_PLUGINS=mymodule.plugin,xdist pytest