[Lock] Added paragraph about auto release

[Nyholm]

This will fix symfony/symfony#40325

@twisted1919 is this paragraph helpful? Should I add something more?

[twisted1919]

I think there should be a note for PCNTL as well, something like:

Please pay special attention when you create a lock and then you call pcntl_fork to create a new process. Resulted process will release the lock when it finishes, leaving the main process unlocked. To prevent this, make sure you do not autoi-release the lock and instead, you release it manually.

Something in between these lines...

Other than this, the info is very helpful, thank you.

[Nyholm]

I've updated the PR to use a better PCNTL example and changed the text to be more clear

[twisted1919]

Looks okay to me, I pointed out a small typo as well, maybe you'll fix that as well.
Thank you :)

[javiereguiluz]

Tobias, thanks for this contribution and for providing a comprehensive example too.

[Nyholm]

Im happy I got all the help from @twisted1919 and the review from @yethee