Document OHTTP relay random selection guidance

[bc1cindy]

Fixed relay order creates a fingerprintable pattern at the network layer, even though OHTTP protects the user's IP. Document that callers should select relays randomly and track failed ones for resilience, following the existing RelayManager implementation in payjoin-cli.

Claude sonnet 4.6 helped me doing the research of the issue.

Closes #1328

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[coveralls]

Pull Request Test Coverage Report for Build 22864083973

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 82.497%

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Totals
Change from base Build 22741440695:	0.0%
Covered Lines:	10638
Relevant Lines:	12895

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💛 - Coveralls

[DanGould]

This is the right direction but raised some questions I don't have the answers for yet about whether the pattern this is documenting is actually a desirable one sufficient for reference.