docs: Fixing P1 formatting problem on Javadocs

The summary sentence for T3PmmFilter didn't end with a period,
which broke the field's line in the summary table, *and* everything
after it in the Javadoc. (See
https://screenshot.googleplex.com/ByBvpYvXjwp.png)

Think this should go in for MR1 DP1 if at all possible.

See first comment for doc stage location.

Bug: 68212949
Test: make ds-docs
Change-Id: I3365c4ce0eaf6a91dde28a4b71249047317fcb5d