Update examples & explanations for input value functions

What does this MR do?

The current wording and examples of Specify functions to manipulate input values and Predefined interpolation functions > expand_vars are misleading.

expand_vars specifically exists for fields where variable expansion usually isn't possible, such as job and stage names. The current examples in both of these documentation sections use expand_vars in the script section, where it is unnecessary.

This MR:

1. Updates the examples so that they demonstrate the use-case for expand_vars
2. Adds an explanation of when to use expand_vars, and when it is not needed.

These changes arose from this Zendesk ticket (internal) where a customer misunderstood how expand_vars worked, thinking that it was required any time you use a variable as an input value:

I'm still not sure what the point of expand_vars is personally. I can't think of a situation where I wouldn't want that to be the default behavior. I think it would be far cleaner to have a function for NOT expanding variables.

Related issues

• How expand_vars works is discussed in this thread: #387632 (comment 1366617361)

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