Update generic field mapping documentation

[Frotty]

Summary

• update the manual to use the finalized MagicFunctions API names
• document explicit-target field iteration and newInstance<T>()
• clarify readonly/constant field behavior, constructor constraints, and same-name user functions
• document compile-time lowering and JASS/Lua parity without runtime reflection

Why

The existing documentation described the earlier __wurst_* prototype and omitted the generic-construction and explicit-target behavior now implemented by WurstScript PR #1223.

Validation

• bin/jekyll build