Man pages matching name mongoc_write_concern_set_fsync in section 3:

mongoc_write_concern_set_fsync.3.en	debian 10 / libmongoc-doc
mongoc_write_concern_set_fsync.3.en	debian 11 / libmongoc-doc
mongoc_write_concern_set_fsync.3.en	debian 12 / libmongoc-doc