Add example to Token! documentation

[ModProg]

This allows rust-analyzer to correctly complete the preferred brackets, instead of parenthesis.

rust-lang/rust-analyzer#15608 (comment)

I wasn't sure what to put in there as the extensive documentation is at the module doc

[dtolnay]

Thanks for the PR!

The example you wrote would get shown to the reader like this:

which is pretty misleading as to how this macro is supposed to be used.

error: macro expansion ends with an incomplete expression: expected one of `!` or `::`
   --> syn-2.0.31/src/token.rs:866:41
    |
866 |     [fn]          => { $crate::token::Fn };
    |                                         ^ expected one of `!` or `::`
    |
   ::: src/lib.rs:1:1
    |
1   | Token![fn]; // syn::token::Fn
    | ---------- in this macro invocation

Could you come up with a more meaningful usage to show in the example?