Fix format string grammar in docs and improve alignment error message #144023

[nilotpal-n7]

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: #144023

[rustbot]

r? @SparrowLii

rustbot has assigned @SparrowLii.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer