Attempt to clarify doc about PlDoc headers

[texttheater]

Avoiding the term "mode" because PlDoc templates don't align 1:1 with modes.
Introduced more explicitly what a template is.

Avoiding the word "must" in instantiation pattern descriptions, phrased things
descriptively.

Added two gotchas, one for users about calling predicates not according to
templates, and one for developers to respect the specific semantics of output
arguments.

[texttheater]

Yep, thanks! Fixed.

[triska]

Great, thank you! Notice also that there is a difference between unbound and free, because a variable may be unbound, and also be involved in constraints. Therefore, instead of:

Determinism declarations assume that the argument is unbound at call. For
the case where the argument is bound, ...

you can say for example:

Determinism declarations assume that the argument is a free variable at call time. For
the case where the argument is bound or involved in constraints, ...

[texttheater]

Ah, I didn't know that, thanks for explaining! Fixed.

[JanWielemaker]

Thanks. I'm not entirely happy with dropping the "must be", but I'm happy enough to have merged
the commits. I squashed them into one though, keeping Markus in the commit message.