Update focus managing section

[theKashey]

A few reasons to update this section:

• Focus and Scroll Locks are very dangerous tools. They could completely "lock" customer, and not let them properly use an application.
• The section does not explain why they could be necessary.
• react-aria-modal was not a great example, as long as it does not work properly on iOS, will prevent operations with portaled context(dropdowns), and does not explain how focus could be managed.

PS: while scroll lock(no-scroll) was a part of react-aria-modal, I am not mentioning it here, as long as this is a wrong section for it.

Reasons to mention react-focus-lock(TLDR - it just works):

• does not prevent "clicking" on elements outside the lock and does not fight with other focus-lock solutions (incidents free)
• works with portals
• works with any possible tabIndexes, and locks scattered to multiple nodes

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