Enhanced BuiltIn.Wait Until Keyword Succeeds doc.

- Explained a bit more explicitly that all normal failures are caught.
- Mentioned that `Fatal Error` can be used to cause a fatal error.

This is related to and closes PR #1817. I do not want to talk about
`Wait Until Keyword Succeeds` in `Fail` docs. That same note should
then be added to all keywords that may fail normally.
Merge branch 'master' of https://github.com/robotframework/robotframework
Documented new --merge option in User Guide.

This finalizes and resolves issue #1687.
Merge branch 'master' of https://github.com/robotframework/robotframework