pallet-revive: Fix docs.rs (#6896)

- Fixed failing docs.rs build for `pallet-revive-uapi` by fixing a wring
attribute in the manifest (we were using `default-target` instead of
`targets`)
- Removed the macros defining host functions because the cfg attributes
introduced in #6866 won't work on them
- Added an docs.rs specific attribute so that the `unstable-hostfn`
feature tag will show up on the functions that are guarded behind it.

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Co-authored-by: command-bot <>