Clarify AsyncIterator use case in template return type guide

The previous description implied AsyncIterator was needed for any async
work, which is wrong -- all template forms support async functions.
The distinguishing characteristic is streaming/dynamic number of outputs.

Author: bcomnes

README.md

@@ -884,9 +884,9 @@ Use the simplest return type that fits your needs:
 | String | No | No (derived from template filename) | Single file, filename from template name |
 | Object | No | Yes | Single file with a custom output path |
 | Array | Yes | Yes | Fixed set of output files known at build time |
-| AsyncIterator | Yes | Yes | Any async work, or a dynamic number of outputs |
+| AsyncIterator | Yes | Yes | Dynamic or unknown number of outputs at build time |
 
-Start with a string return and only switch to a more complex type when you need what it provides. An async generator is the right choice when you need to do async work (such as calling `renderInnerPage` on multiple pages) and produce more than one output file.
+Start with a string return and only switch to a more complex type when you need what it provides. All template forms can do async work (string, object, and array all support `async` functions). Choose AsyncIterator specifically when the number of output files is not known until the template runs, or when you want to stream outputs one at a time rather than building the full list in memory first.
 
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