Document remaining memcached constants on php.net?

[TysonAndre]

e.g. php/doc-en#584 (noreply seems to work as expected when manually testing it, though it is not covered by phpt tests). php.net documentation now only needs to cover php 7.0+ behavior and changes.

Many other constants in https://github.com/php-memcached-dev/php-memcached/blob/master/memcached-api.php haven't been documented

https://github.com/php-memcached-dev/php-memcached/blob/master/php_memcached.c has many constants corresponding to MEMCACHED_BEHAVIOR_*

Related to #343