[PR #14113/6d0aff14 backport][9.0.x] Docs: clarify -p vs PYTEST_PLUGINS plugin loading

[patchback]

This is a backport of PR #14113 as merged into main (6d0aff1).

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Summary

This PR clarifies the difference between -p and PYTEST_PLUGINS in the plugin documentation and fixes an incorrect command example.

Details

• Fixes the incorrect example pytest --disable-plugin-autoload -p NAME,NAME2
  → replaced with the correct repeated form -p NAME -p NAME2
• Clarifies the different use-cases:
  • -p early-loads (or disables) a plugin by name or entry point for a single invocation
  • PYTEST_PLUGINS loads plugin modules during startup, commonly used when testing plugins
• Adds guidance to avoid duplicate plugin registration when plugin autoloading is disabled
• Adds concrete examples for both mechanisms

closes #13388
Changelog: not needed (documentation-only change).