Point out in docs that "Cache HTTP headers with page content." will disable static full page caching

The option **Cache HTTP headers with page content**, when checked, will disable static full page cache.

Reasons are (now) obvious to me, but it should be point out in a one-line description placed below the label.

Start by locating the documentation or settings UI entry for “Cache HTTP headers with page content.” Add the requested one-line explanation below its label, stating that enabling it disables static full page caching. Done when the warning is visible next to the option and the surrounding wording remains clear.

Written by the indexing model from the issue text.