Add documentation for the `max_srcset_image_width` filter.

[joemcgill]

In version 3.0 (and in WP 4.4), we added a low-pass filter to the sizes that will be included in srcset attributes, which defaults to 1600px, but is filterable using the max_srcset_image_width hook (link).

We need to add documentation for this hook with code samples for filtering conditionally based on the width of the original image.

[jaspermdegroot]

Landed in 81ab3c8590a2679fc7cab16e92736857d82a34e7