docs: explain how to remove fish help abbreviations

[SergioChan]

Summary

• document how fish users can remove accidental help abbreviations created for bat --help / bat -h
• add copy-paste erase commands for both long and short forms
• keep the change scoped to the shell-aliases section in README.md

Testing

• git diff --check upstream/master...HEAD
• Rust toolchain is not available in this environment, so runtime tests were not runnable for this docs-only update.

Related

Fixes #3536

[SergioChan]

Added follow-up commit 1ad4dd3 to satisfy the required changelog check.

What changed

• Added one unreleased changelog line:
  • Document how to remove fish help abbreviations, see #3624 (@SergioChan)

No behavior/code changes beyond this PR’s existing README docs update.