Rollup merge of rust-lang#146645 - yotamofek:pr/rustdoc/fndecl_inner_full_print, r=fmease

Cleanup `FnDecl::inner_full_print`

`inner_full_print` was pretty hard to follow IMHO.
Hopefully this cleans it up a little bit.

Also, it was checking whether `self.inputs` is empty twice, and then handling an unreachable match arm:
https://github.com/yotamofek/rust/blob/f836ae4e663b6e8938096b8559e094d18361be55/src/librustdoc/html/format.rs#L1368C1-L1368C33
`last_input_index` could only be `None` if the fn has no parameters, in which case the loop body would never run.

r? ``@GuillaumeGomez`` if you have the capacity :)

BTW, can we rename `FnDecl::inputs` to `parameters` or something? And `output` to `return_ty`?

Author: Zalathar