docs: add toBeVisible matcher aria-hidden note

[opichals]

What:

Add README docs on toBeVisible matcher not checking the aria-hidden attribute.

Why:

For consistency with the aria-disabled note added earlier in #303

Checklist:

N/A

[codecov]

Codecov Report

Merging #398 (83074aa) into main (d24b6be) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #398   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           26        26           
  Lines          615       615           
  Branches       223       225    +2     
=========================================
  Hits           615       615           

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