Update BeOp doc to add User Syncs reference

[sebrobert]

Add mention of User Sync and missing features

🏷 Type of documentation

• new bid adapter
• update bid adapter
• new feature
• text edit only (wording, typos)
• bugfix (code examples)
• new examples

📋 Checklist

• Related pull requests in prebid.js
• For new adapters check submitting your adapter docs

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[bretg]

This looks related to prebid/Prebid.js#12944.

Couple of questions/requests:

1. please consider adding the many fields missing from your adapter's header as described in https://docs.prebid.org/dev-docs/bidder-adaptor.html#submitting-your-adapter

e.g.

dsa_supported: true/false
usp_supported: true/false
coppa_supported: true/false
gpp_sids: tcfeu, tcfca, usnat, usstate_all, usp
schain_supported: true/false
dchain_supported: true/false
userId: (list of supported vendors)
media_types: banner, video, native
safeframes_ok: true/false
deals_supported: true/false
floors_supported: true/false
fpd_supported: true/false
pbjs: true/false
pbs: true/false
prebid_member: true/false
multiformat_supported: will-bid-on-any, will-bid-on-one, will-not-bid
ortb_blocking_supported: true/partial/false
privacy_sandbox: no or comma separated list of `paapi`, `topics`

2. Your 'currency' parameter is quite unfriendly to publishers. They declare their auction currency using the currency module. You module should look at that and decide which currency to bid in.