Fix format string grammar in docs and improve alignment error message for #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? @lcnr

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[lcnr]

r=me after updating PR description + squashing commits

View changes since this review

[rustbot]

Some changes occurred in src/tools/cargo

cc @ehuss

[lcnr]

You've accidentally committed changes to submodules here; see the above comment added by rustbot. This is something that can very easily happen, so don't worry about it :> still need to fix it though

[nilotpal-n7]

You've accidentally committed changes to submodules here; see the above comment added by rustbot. This is something that can very easily happen, so don't worry about it :> still need to fix it though

Ah, got it 🙂.
What I did was: I synced the latest master, ran ./x build, then checked out my fix-format-issue branch (without syncing it) and made changes there before pushing. Looks like that pulled in a submodule update accidentally.
Is that the wrong way to do it? Or should I always rebase my feature branch on top of the updated master instead?

[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.

[lcnr]

Please also squash this PR/merge it all into 1 commit. It's not large enough for 6 commits 😁 what tend to use is

 git rebase -i --autosquash $(git merge-base HEAD origin/master)

and then merge all relevant commits by changing them from pick to f (you can also move them around, git should provide an overview of what you can do with these commits in the relevant file)