Fix UInt unary negation ScalaDoc (backport #4829)

[chiselbot]

Fixing a ScalaDoc inconsistency noted on Element.

Note that unary operators are only allowed to be single characters, -, !, ~ and +, so unary_-% has always been invalid to call as a unary operator, you have to call it as myUInt.unary_-% which is just pointless and silly.

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
• Did you specify the type of improvement?
• Did you add appropriate documentation in docs/src?
• Did you request a desired merge strategy?
• Did you add text to be included in the Release Notes for this change?

Type of Improvement

• API deprecation

Desired Merge Strategy

• Squash

Release Notes

Deprecate invalid unary operator unary_-%.

Reviewer Checklist (only modified by reviewer)

• Did you add the appropriate labels? (Select the most appropriate one based on the "Type of Improvement")
• Did you mark the proper milestone (Bug fix: 3.6.x, 5.x, or 6.x depending on impact, API modification or big change: 7.0)?
• Did you review?
• Did you check whether all relevant Contributor checkboxes have been checked?
• Did you do one of the following when ready to merge:
  • Squash: You/ the contributor Enable auto-merge (squash) and clean up the commit message.
  • Merge: Ensure that contributor has cleaned up their commit history, then merge with Create a merge commit.

---

This is an automatic backport of pull request #4829 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 477900b has failed:

On branch mergify/bp/6.x/pr-4829
Your branch is up to date with 'origin/6.x'.

You are currently cherry-picking commit 477900b9.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   core/src/main/scala-2/chisel3/BitsIntf.scala
	deleted by us:   core/src/main/scala-3/chisel3/BitsIntf.scala

no changes added to commit (use "git add" and/or "git commit -a")

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