docs: Fixed summary fragment and removed typo

The CommonTypos linter complained about the typo "for for". While I had
the file open, also changed the first sentence to be a proper summary
fragment, per http://go/java-style#s7.2-summary-fragment.

No code changes, just docs.

Bug: 263824202
Change-Id: Iea27f1d2a91918a253e099365ebbd2daa9bcb3cd
Test: [go/abtd docs build]