gpui_macros: Disable doctests (#13015)

This PR disables the doctests in the `gpui_macros` crate, as they depend
on `gpui` to run.

Since `gpui` depends on `gpui_macros`, we don't really want to add a
dependency on `gpui` (even though it _appears_ to work as a dev
dependency).

Also did some minor stylistic cleanup of some doc comments.

Release Notes:

- N/A

crates/gpui_macros/Cargo.toml

@@ -11,6 +11,7 @@ workspace = true
 [lib]
 path = "src/gpui_macros.rs"
 proc-macro = true
+doctest = false
 
 [dependencies]
 proc-macro2 = "1.0.66"