Document htmlIdGenerator use cases

[cjcenizal]

Some questions arose in #637 which we should answer with documentation. I think we need to add a page for htmlIdGenerator under the "Utilities" section of the docs site, which presents realistic examples of the use cases @timroes mentioned in #637 (comment):

• One generator used to create a one-off ID, e.g. htmlIdGenerator()()
• One generator using a prefix to create a series of IDs that are namespaced, e.g. a form component which will be instantiated in multiple places within the same UI
• Two different generators using the same suffix, resulting in unique IDs
• A generator using both a prefix and a suffix, though I'm not sure if there is a use case for this?