Update docs for image caching behavior (#29895)

Revises docs to explain that the maximum of the header and config is used.

docs/api-reference/next/image.md

@@ -318,12 +318,13 @@ The following describes the caching algorithm for the default [loader](#loader).
 
 Images are optimized dynamically upon request and stored in the `<distDir>/cache/images` directory. The optimized image file will be served for subsequent requests until the expiration is reached. When a request is made that matches a cached but expired file, the cached file is deleted before generating a new optimized image and caching the new file.
 
-The expiration (or rather Max Age) is defined by the upstream server's `Cache-Control` header.
+The expiration (or rather Max Age) is defined by either the [`minimumCacheTTL`](#minimum-cache-ttl) configuration or the upstream server's `Cache-Control` header, whichever is larger. Specifically, the `max-age` value of the `Cache-Control` header is used. If both `s-maxage` and `max-age` are found, then `s-maxage` is preferred.
 
-- If `s-maxage` is found in `Cache-Control`, it is used. If no `s-maxage` is found, then `max-age` is used. If no `max-age` is found, then [`minimumCacheTTL`](#minimum-cache-ttl) is used.
-- You can configure [`minimumCacheTTL`](#minimum-cache-ttl) to increase the cache duration when the upstream image does not include `max-age`.
-- You can also configure [`deviceSizes`](#device-sizes) and [`imageSizes`](#device-sizes) to reduce the total number of possible generated images.
-- You can also configure [formats](/docs/basic-features/image-optimization.md#acceptable-formats) to disable multiple formats in favor of a single image format.
+- You can configure [`minimumCacheTTL`](#minimum-cache-ttl) to increase the cache duration when the upstream image does not include `Cache-Control` header or the value is very low.
+- You can configure [`deviceSizes`](#device-sizes) and [`imageSizes`](#device-sizes) to reduce the total number of possible generated images.
+- You can configure [formats](/docs/basic-features/image-optimization.md#acceptable-formats) to disable multiple formats in favor of a single image format.
+
+### Minimum Cache TTL
 
 You can configure the Time to Live (TTL) in seconds for cached optimized images. In many cases, it's better to use a [Static Image Import](/docs/basic-features/image-optimization.md#local-images) which will automatically hash the file contents and cache the image forever with a `Cache-Control` header of `immutable`.