Fix format string grammar in docs and improve alignment error message for #144023
This PR improves error messages and documentation for format strings involving alignment and formatting traits.
Highlights:
- Clearer error messages for invalid alignment specifiers (e.g., `{0:#X>18}`), showing the expected `<`, `^`, or `>` and a working example:
println!("{0:>#18X}", value);
- Updated UI test `format-alignment-hash.rs` to reflect the improved error output.
- Documentation clarification: ensures examples correctly show how width, alignment, and traits like `x`, `X`, `#` combine.
Motivation:
Previously, using `#` with alignment and width produced confusing errors. This PR guides users on the correct syntax and provides actionable examples.
Testing:
- Built the compiler (`./x build`)
- Blessed and ran UI tests (`./x. test src/test/ui/fmt/format-alignment-hash.rs --bless`)
- Verified full test suite passes (`./x test`)
Issue: rust-lang/rust#144023
e5f96e3b43