docs: add instructions for removing fish help abbreviations

Add documentation showing how to erase the fish abbreviations for
--help and -h, since the dash-prefixed names make removal non-obvious.
The key is using -- before the abbreviation name to prevent fish from
interpreting it as a flag.

Closes #3536

CHANGELOG.md

@@ -1,6 +1,10 @@
 # unreleased
 
 
+## Other
+
+- Add instructions for removing fish help abbreviations to README, see #3655 (@claw-explorer). Closes #3536
+
 ## Features
 
 - Preserve `--diff` change markers and snip separators when `--plain` is set. Closes #3630, see #3643 (@mvanhorn)