doc: httpd::file_handler: document undocumented parameters
doc: httpd::file_handler: document undocumented parameters

Resolves following warning:
include/seastar/http/file_handler.hh:153: warning: The following parameters of seastar::httpd::file_handler::file_handler(const sstring &file, file_transformer *transformer=nullptr, bool force_path=true) are not documented:

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

dreamsxin/seastar,syuu1228/seastar,avikivity/seastar,cloudius-systems/seastar,dreamsxin/seastar,syuu1228/seastar,cloudius-systems/seastar,scylladb/seastar,avikivity/seastar,dreamsxin/seastar,scylladb/seastar,avikivity/seastar,syuu1228/seastar,cloudius-systems/seastar,scylladb/seastar