docs: allow the export_test.go alias

[retr0h]

Follows osapi-io/specs#80, which corrected the export_test.go requirement.

This repository stated the strict form in two places — the test file conventions and done-definition item 6 — saying export_test.go may expose setters only and never var ReadX = readX aliases.

That contradicts the corrected capability. The alias is idiomatic Go; twenty-two standard library packages use it, and this repository has four of its own (BytesToString, DefaultUname, ReadInterfaces, DefaultInterfaceAddrs) that the strict rule made violations of its own documentation.

Both passages now state what the rule was always protecting:

• Do not use an alias to test an internal step the caller's own test already covers. A pure helper with its own contract is what the pattern is for.
• Seams sit at the boundary with the upstream library, so the bridge mapping runs on every table row — not partway through the collector.

just md-fmt-check passes.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff            @@
##              main      #161   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          184       184           
  Lines         6359      6359           
=========================================
  Hits          6359      6359           

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