Add info telling that ownCloud only supports InnoDB for MySQL (not MyISAM)

[PVince81]

In OC 7 there was a PR that introduced a repair step that converts MyISAM tables to InnoDB: owncloud/core#9492

We should add a hint in the documentation that MyISAM isn't supported. There are some shared hosters that do not support InnoDB.

Also maybe add a hint in the setup check section?

@RealRancor @DeepDiver1975 @icewind1991 @felixboehm

[felixboehm]

This should be checked during install. Maybe the admin should verify that he really wants to install to MyISAM?

[ghost]

@felixboehm Probably needs a new issue in https://github.com/owncloud/core/issues as such a request gets lost in the documentation tracker

@PVince81 PR in #2501