Show rustdoc of proc macros

Thank you for rust-analyzer.

• Please, consider showing rustdoc of attribute macros (on mouse over in VS Code...).
• And, if you do choose to implement it, please also carry rustdoc if a macro is imported through use ... as ...; under an alias.

Example: Attribute macros aliased on purpose from https://docs.rs/allow_prefixed/latest/allow_prefixed (or from https://docs.rs/allow_prefixed/latest/allow):

• https://github.com/coop-rs/allow/blob/70e78689b56ff3d19a61f42053999713c281bcc2/Cargo.toml
• https://github.com/coop-rs/allow/blob/70e78689b56ff3d19a61f42053999713c281bcc2/src/lib.rs

Crrent rust-analyzer version v0.3.1506 in current VS Code:

Version: 1.78.1
Commit: 6a995c4f4cc2ced6e3237749973982e751cb0bf9
Date: 2023-05-04T09:46:23.602Z
Electron: 22.5.1
Chromium: 108.0.5359.215
Node.js: 16.17.1
V8: 10.8.168.25-electron.0
OS: Linux x64 6.3.0-1-MANJARO
Sandboxed: No

Thank you in advance.

Start by reproducing the attribute-macro example from allow_prefixed, using its Cargo.toml and src/lib.rs, in the reported VS Code and rust-analyzer setup. Trace the hover or rustdoc behavior for the macro and its use ... as ... alias; done means both direct and aliased attribute macros show their rustdoc.