Document entry_point in cosmwasm-std instead of derive (backport #2245)

[mergify[bot]]

Closes #2186

Turns out we can document the re-export instead of the function definition. This also executes the doc tests as part of cosmwasm-std.

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This is an automatic backport of pull request #2245 done by Mergify.

[mergify[bot]]

Cherry-pick of e230048f13e25982371ca1c3c7f97c0e87aa08c6 has failed:

On branch mergify/bp/release/2.0/pr-2245
Your branch is up to date with 'origin/release/2.0'.

You are currently cherry-picking commit e230048f1.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
    both modified:   Cargo.lock
    both modified:   packages/derive/Cargo.toml
    both modified:   packages/derive/src/lib.rs
    both modified:   packages/std/src/lib.rs

no changes added to commit (use "git add" and/or "git commit -a")

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