[codex] Clarify streaming interpolation macros

[bobzhang]

Summary

Clarifies the MoonBit guide section for <+ and <? streaming interpolation macros.

Details

• Shows valid <? right-hand-side forms instead of an arbitrary identifier expression.
• Documents that <? skips writes for None and applies the <+ expansion for Some(writer).
• Calls out the additional writer methods required for map object literals: write_object_begin, write_object_field, and write_object_end.

Validation

• git diff --check
• moon run -e '...' snippet covering <+ template strings, <+ map object literals, <? template strings, <? map object literals, and None no-op behavior.