> But users should preferably not need to worry
> about variable interactions.  The doc for a given
> variable should make clear just what it does, and
> each variable should preferably have one behavior
> (per value chosen).

I agree that it's better to make it clear in the docstring.
Fixed in a new patch below.

> I'm guessing that nil `search-exit-option' does
> not just have "the same effect".  But (see above)
> even if it does, that doesn't mean that option
> `search-exit-option' has the same effect, because
> setting it to non-nil, ONLY to NOT have the
> effect of `unlimited' `isearch-allow-scroll',
> would presumably also have some other effect
> unrelated to allowing scrolling.

I agree that `search-exit-option' is too confusing variable
for such features.  So we have to offload it from all
unrelated features.

As the first step, I moved the recently added shift-select
feature from `search-exit-option' to its own clearly named
customizable variable `isearch-allow-shift-select'.

For the same reason, unlimited scrolling was moved to the new option
`unlimited' of `isearch-allow-scroll'.

Now finally everything looks right.  Please try a new patch: