http: clarify the ownership in the router's doxygen comment

[kreuzerkrieg]

Update the routes's put, add, add_default_handler, add_cookie and del_cookie doxygen documentation

[tchaikov]

the title of the commit message reads:

Update the routes's put, add, add_default_handler, add_cookie and del_cookie doxygen documentation.

update ... documentation, is not accurate enough, i'd suggest put:

http: clarify the ownership in the router's doxygen comment

for two reasons:

• add the prefix of the area we are changing. please note, we do have a "doc" directory in seastar.
• be more specific on the change

[tchaikov]

@scylladb/seastar-maint hello maintainers, could you help review this change?