Rollup merge of #143909 - nik-rev:patch-1, r=jhpratt

docs(alloc::fmt): Make type optional, instead of matching empty string

Think this is clearer.

Noticed as I was implementing [`tree-sitter-rust-format-args`](https://github.com/nik-rev/tree-sitter-rust-format-args), and attempting to match the empty string results in an error.