Document SpiderMonkey GC Rust and Wasm boundaries

[teerthsharma]

Summary

• document how Rust ownership differs from SpiderMonkey GC-managed JS/Wasm values
• clarify that Rust/Wasm bridges must keep SpiderMonkey responsible for tracing, barriers, weak edges, and cleanup scheduling
• describe topology-based GC instrumentation as disabled-by-default observation, not collector replacement

Verification

• git diff --check
• lightweight RST content assertions

./mach doc --no-serve --no-open was attempted locally but could not start because MozillaBuild was not found at C:\mozilla-build.

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