Twenty Seventeen: Filters are not documented

[ocean90]

Twenty Fifteen has 5 filters and 4 are documented, Twenty Sixteen has 8 filters and 6 are documented.
Twenty Seventeen has 4 filters and none are documented.

See ​https://make.wordpress.org/core/handbook/best-practices/inline-documentation-standards/php/#4-hooks-actions-and-filters

[brainstormforce]

Patch adds doc blocks for filters in Twenty Fifteen and Twenty Seventeen

[davidakennedy]

In 38846:

Twenty Fifteen, Twenty Seventeen: Add documentation to filters in themes.

Documents a missing one Twenty Fifteen and all four missing ones in Twenty Seventeen.

Props brainstormforce.

Fixes #38382.