[Docs] Clarify that "current passage" means the current moment, not the passage the code is running from within.

[ChapelR]

For example, the tags() function features this line:

If omitted, will default to the current passage.

Some users have interpreted this to mean the passage the code is in rather than the passage associated with the current moment, e.g., they believe a tags() call in PassageHeader would return the tags of the PassageHeader passage.

I do not think the docs as written are necessarily unclear, but I think it wouldn't hurt to specify what is meant by the "current passage."

[tmedwards]

Ending up making two commits addressing this—one for the argument text and a another later for the example. Better?

[ChapelR]

Looks good to me.