Clarify language in 8.3.5 <condition>.

[skynavga]

Improve language by changing

If a <condition> value contains a function-expression non-terminal, then it must take the form of a expression.

to

A <function-expression> that appears in a <condition> value must take the form of a <condition-function> expression.

[nigelmegitt]

It'd be useful to explain the motivation for this change. Both the existing and proposed variants do not seem problematic. The phrase "non-terminal" doesn't seem to be helpful, so happy to remove it.

[nigelmegitt]

See motivation from @skynavga at #996 (comment) :

@palemieux thought 'it' could refer to <condition> (I didn't) and didn't like 'non-terminal' either