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]

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[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[rustbot]

⚠️ Warning ⚠️

• The following commits have merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

  • c23a1b2

  You can start a rebase with the following commands:

  $ # rebase
  $ git pull --rebase https://github.com/rust-lang/rust.git master
  $ git push --force-with-lease