doc: fix doc styles

Summary

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CONTRIBUTING.md

• L857: Unused definition
• L861: Unused definition
• L863: Unused definition

doc/api/assert.md

• L719: Unused definition

doc/api/async_hooks.md

• L460: Missing code-language flag

doc/api/child_process.md

• L1362: Unused definition

doc/api/dns.md

• L674: Unused definition

doc/api/esm.md

• L178: Missing code-language flag

doc/api/http.md

• L1868: Unused definition
• L1887: Unused definition
• L1888: Unused definition
• L1889: Unused definition
• L1916: Unused definition
• L1917: Unused definition

doc/api/https.md

• L260: Unused definition

doc/api/os.md

• L1226: Unused definition

doc/api/process.md

• L1888: Unused definition

doc/api/stream.md

• L2227: Definitions with the same identifier

doc/guides/writing-and-running-benchmarks.md

• L1: Missing newline character at end of file

See also

Refs: https://github.com/nodejs/node/pull/12756

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc