docs(language): remove obsolete limitations

[TheHypnoo]

Summary

• Remove obsolete limitations for Symbol, computed property names, and advanced Error support from the language limitations page.
• Keep the remaining caveats focused on actual limitations: runtime metadata, user-space CommonJS require, prototype mutation, GC-accurate weak references, and non-universal Proxy trapping.
• Update the limitations doc fixture comment so it matches the remaining intentionally rejected examples.

Validation

• git diff --check
• target/debug/perry docs/examples/language/limitations.ts -o /tmp/perry_limitations/docs_limitations_example && /tmp/perry_limitations/docs_limitations_example