Provide a better UX for users considering sharing their contact list with an IS to discover people they know already using Matrix

As per https://github.com/vector-im/riot-ios/issues/2521, the current UX has a static message which does an okay job of telling users what will happen. It would be better if we could:
- say for certain whether personal data would be hashed before lookup
- direct users where they can review the IS policy docs before giving the app permission to access the address book

There's a possibility this could be solved in part by https://github.com/vector-im/riot-ios/issues/2710

Read the related Riot iOS issues 2521 and 2710, then locate the current static message for contact-list sharing. Verify what the identity server does with personal data and identify the policy documentation users should review; done means the consent UX clearly states hashing and links to those policy docs.

Written by the indexing model from the issue text.