Update cache documentation (#1622)

* Fix cache custom settings

The nhibernate session factory configuration settings is restricted by a
xsd. Adjust documentation of custom settings for reflecting this.

* Adjust caches documentation

  * Fix typos
  * Remove obsolete stuff
  * Add a note on caches potentially shared between session factories
  * Link each cache to its configuration documentation
  * Give ids to all sections
  * Switch Memcache for EnyimMemcached and document it. The former is
    no more maintained since years, and its NuGet package is not updated.
    Moreover it fails some basic tests.
    The later appears to be no more maintained since one year and a half,
    but it does not fail tests and its NuGet package is still updated.

* Document CoreMemoryCache

* Document CoreDistributedCache

Author: fredericDelaporte