incorrect description of return value in DocBlock of WP_List_Table::handle_row_actions()

[pbiron]

Currently the text reads:

The row actions HTML, or an empty string if the current column is the primary column.

and it should read:

The row actions HTML, or an empty string if the current column is not the primary column.

[SergeyBiryukov]

In 47058:

Docs: Correct @return description for WP_List_Table::handle_row_actions().

Props pbiron.
Fixes #49170.

[SergeyBiryukov]

In 47059:

Docs: Synchronize @return descriptions for ::handle_row_actions() methods in list tables.

Make sure WP_Comments_List_Table::handle_row_actions() and WP_MS_Sites_List_Table::handle_row_actions() return a string, for consistency with other classes.

See #49170, #48303.