[BugFix][UMA] Protect target registration (apache#13624) (#3)

This PR address fixes for UMA target registration.
* Fix the doc issue apache#13304
* Continues stalled PR apache#12731

Changes:
* Incorporates all proposed fixes from mentioned [PR apache#12731](apache#12731)
* Address test case concerns and discussions from [PR apache#12731](apache#12731)
* **NEW:** Already exiting target cannot be created, explicit error on this.
* **NEW:** Attributes having special/reserved scope cannot be created explicitly.

It also address proper test cases for all the above.

Signed-off-by: tqchen <[email protected]>
Co-authored-by: Balint Cristian <[email protected]>