Document the PHP 8.5 core language features

[lacatoire]

Documents four PHP 8.5 core features still marked Todo on the 8.5 documentation tracker.

• Closures and first-class callables in constant expressions — PHP 8.5 Documentation Tracker (view)
• Casts in constant expressions — PHP 8.5 Documentation Tracker (view)
• Closure is now a proper subtype of callable — PHP 8.5 Documentation Tracker (view)
• Fatal errors now include a backtrace — PHP 8.5 Documentation Tracker (view)

Wording follows the PHP 8.5 UPGRADING notes; each entry was checked against PHP 8.5.4.