Wrong version numbers used in the User Guide and in a deprecation warning

[pekkaklarck]

We decided to change version number of RF 5.1 to RF 6.0 pretty close to the final release. I tried to update docs and code so that they'd use correct version numbers, but missed few places:

• The User Guide talks about deprecating Force Tags and Default Tags in RF 6.0 when in fact that will happen (earliest) in RF 7.0.
• The User Guide explains that -tag syntax can be used for removing tags in RF 5.2 but that's going to happen in RF 6.1.
• A deprecation warning about using -tag syntax also says that behavior will be changed in RF 5.2.