Document entry_point in cosmwasm-std instead of derive

[webmaster128]

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.

[chipshort]

Interesting, I tested this before and thought it didn't work because it does not show up when hovering in vscode. But it does work with cargo doc, so I think we can go with it.

[webmaster128]

In https://doc.rust-lang.org/rustdoc/write-documentation/re-exports.html it is explained how all of this is supposed to work. I did not really understand all the cases but it seems to be well defined behaviour.

[aumetra]

@mergify backport release/1.5 release/2.0 release/2.1

[mergify]

backport release/1.5 release/2.0 release/2.1

✅ Backports have been created

• #2265 Document entry_point in cosmwasm-std instead of derive (backport #2245) has been created for branch release/1.5 but encountered conflicts
• #2266 Document entry_point in cosmwasm-std instead of derive (backport #2245) has been created for branch release/2.0 but encountered conflicts
• #2267 Document entry_point in cosmwasm-std instead of derive (backport #2245) has been created for branch release/2.1 but encountered conflicts