Wrong return type in comment docs of {theme}_post_thumbnail_sizes_attr function

[truongwp]

In the comment docs of twentysixteen_post_thumbnail_sizes_attr() and twentyseventeen_post_thumbnail_sizes_attr(), the return type should be array instead of string.

[SergeyBiryukov]

In 41341:

Twenty Sixteen: Correct the @return value for twentysixteen_post_thumbnail_sizes_attr().

Props truongwp.
See #41830.

[SergeyBiryukov]

In 41342:

Twenty Seventeen: Correct the @return value for twentyseventeen_post_thumbnail_sizes_attr().

Props truongwp.
Fixes #41830.