Document procedural macros in expression position

[Huzaifa-Asif]

Summary

• extend the function-like procedural macro example with a macro that expands to the expression 42
• invoke that macro directly in expression position alongside the existing item-position example

This fills the documentation gap described in #858 without changing the surrounding normative text.

Closes #858.

Verification

• git diff --check — passed locally
• GitHub CI — code tests, style tests, mdbook-spec, tools, preview, dev-guide build, zizmor, and success gate all passed
• Local cargo xtask style-check could not start because interrupted rustup downloads left the local nightly and stable installations with missing manifests

AI assistance disclosure

OpenAI Codex assisted with issue research, the documentation edit, verification, and drafting this description.