Add tip when index searches bigger than 1

[gleu]

The new "Index searches" info in v18 show how many scan of the index happened. If there's more than 1, it means it was a skip scan. Adding "Skip scan" explicitly would be helpful.

[gleu]

See https://www.robins.in/2026/02/the-skip-scan-you-already-had-before-v18.html for more info.

[gleu]

Looks like I was over optimistic: https://www.pgmustard.com/blog/what-do-index-searches-in-explain-mean