Update API reference docs before 1.0.0 stable

[bartekn]

• Make sure that API docs are up to date with all the latest changes.
• Make sure all the endpoints have detailed explanation of the data.

  • /assets count return authorized trust lines only

  • /accounts returns all trust lines (authorized and unauthorized)

  • Update /fee_stats response.

  • Remove deprecated fee_paid field from Transaction response (replace it with the fee_charged and max_fee fields)

  • Update fields from int64 to string.

    • Attribute offer_id in manage buy offer and manage sell offer operations.

    • Attribute offer_id in Trade effect.

    • Attribute id in Offer resource.

    • Attribute timestamp and trade_count in Trade Aggregation resource.

    See Decide on use of int64 in JSON responses. #1609, protocols/horizon/effects: Add UnmarshalJSON to SequenceBumped. #1909, and Support String or Number Unmarshalling in Int64 fields #1912 for more info

  • Update operation types to canonical names (if needed) (see protocols/horizon: Apply deprecation on some operation types, updating them to the correct value #2134).

[abuiles]

@bartekn I just updated a bunch of docs in #2253, added as checklist here, please check it out, let me know if something is missing and I can fix it.

[abuiles]

Fixed, the release branch has all the docs up to date.