Doc: Correct FAQ explanations - #161663

• Clarify that atexit runs cleanup functions when Python exits, rather than when objects are deallocated.
• Replace the obsolete extension-loading breakpoint with GDB pending breakpoints, allowing users to stop directly in the function they want to debug.
• Describe reversed() as returning a reverse iterator, rather than copying the sequence.
• Explain that super() returns a proxy that searches the MRO, rather than the next class itself.
• Correct self.count to be an instance attribute, rather than a new instance.
• Remove the mutable Weather caching example: changing the hash of an object already used in a cache key violates hashability requirements and can return stale results.

Validation: built the HTML documentation, ran git diff --check, and verified that a pending GDB breakpoint on math_sin stops when the dynamically loaded function is called.