Clarifications around tasks, groups, and the conditional terminator `--` in SBY docs

[gussmith23]

A few things that I think could use clarification in SBY docs, based on issues I actually encountered.

• It seems like task names and group names can be used interchangeably in some places, though the documentation doesn't make that super clear. For example, these lines:

  If the tag : is used on a line by itself then the conditional string extends until the next conditional block or -- on a line by itself.

  and

  The tag ~: can be used for a line or block that should not be used when the given task is active:

  Unless it really is the case that the above only works with tasknames and not groupnames.

• The line about the terminator -- may be technically correct, but I think we should more clearly state how the user should use it. Currently it says:

  If the tag : is used on a line by itself then the conditional string extends until the next conditional block or -- on a line by itself.

  It might be clearer to just provide explicit guidance saying that, if the user uses conditionals in a block, then the block must be terminated by a --.

[KrystalDelusion]

Re tags/tasks, the two terms are used interchangeably, as I understand it tasks are the things that SBY runs (there is only ever one current task), but a task can have multiple tags and one of those tags is the name of the task. But yeah it's not super clear in the documentation.