doc: update

[kbond]

EDIT (from @nikophil)

I think the doc on symfony.com is updated as soon as the doc is modified (disregarding releases). So, here is the plan for 2.x:

• finish and validate this PR without merging it
• once validated, copy this PR into another which will target 2.x and add missing pieces
• merge both doc PR + release both 1.x and 2.x 🚀

todo for 2.x documentation:

• document ArrayFactory
• document __index for factory collections this was already documented
• make:factories: document --with-phpdoc option and add change the note about @method
• remove documentation of the config auto_refresh_proxies

[nikophil]

something else: WDYT about documenting this little, but still useful, behavior (the stuff with __index)?

[kbond]

Thanks for making these changes @nikophil!

Your plan for syncing this with 2.x makes sense. We'll need to update https://github.com/zenstruck/foundry/blob/1.x/.symfony.bundle.yaml (probably on 1.x and 2.x)