memcache already has the new API Version 3

[liyimingcust]

From manual page: https://php.net/class.memcache

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see https://github.com/websupport-sk/pecl-memcache
New API in 3.0
The parameter definitions of some interfaces have changed.Better update the doc

[cmb69]

See also websupport-sk/pecl-memcache#67.

It would be helpful if the memcache extension would provide stub files; that would allow us to at least add some skeleton documentation, which could be fleshed out over time. Plus, it would simplify things for the extension maintainers in the long run.