Document asm_cfg inline assembly attributes (review feedback)

[PLeVasseur]

Summary

• Document stabilized asm_cfg support for inline assembly templates and operands, with grammar and legality updates in src/inline-assembly.rst.
• Apply review-driven clarifications: remove syntax-redundant optionality prose, replace undefined umbrella wording with explicit entities, make the attribute allowlist normative, and make cfg-false/template-first behavior explicit.
• Keep changelog paragraph references in sync with the final paragraph set in src/changelog.rst.

Closes rust-lang#647

Reference alignment

• Reference PR: document cfg conditions on inline assembly templates and operands rust-lang/reference#2063
• Rust PR: Stabilize asm_cfg rust-lang/rust#147736
• Reference sections: https://doc.rust-lang.org/reference/inline-assembly.html#syntax and https://doc.rust-lang.org/reference/inline-assembly.html#attributes
• Alignment: semantics remain aligned with Reference/rustc behavior (cfg/cfg_attr support and template-first parsing constraint); this update tightens FLS wording for precision and normative clarity.
• FLS-specific grammar naming remains: AssemblyTemplate and AssemblyAttribute* wrappers around existing operand categories.

Testing

• ./make.py