/// Extracts the item with specified \p key
/** \anchor cds_nonintrusive_MichaelHashMap_hp_extract
The function searches an item with key equal to \p key,
- unlinks it from the set, and returns it as \p guarded_ptr.
+ unlinks it from the map, and returns it as \p guarded_ptr.
If \p key is not found the function returns an empty guarded pointer.
Note the compare functor should accept a parameter of type \p K that may be not the same as \p key_type.