doc: Manage expectations for eval-cache

Incorrectly high expectations lead to frustration for users who
stick around to experience how useless it is for e.g. a devShell

https://functional.cafe/@arianvp/112976284363120036:

> Flakes doesn't have eval caching. It has command line argument
> caching. It literally just stores the cli argument you passed
> in a sqlite database and yes that's as useless as it sounds

> When I discovered flakes had no expression level caching whatsoever
> I kind of felt lied to and betrayed.

doc/manual/src/release-notes/rl-2.4.md

@@ -141,6 +141,8 @@ more than 2800 commits from 195 contributors since release 2.3.
   the evaluation cache. This is made possible by the hermetic
   evaluation model of flakes.
 
+  Intermediate results are not cached.
+
 * The new `--offline` flag disables substituters and causes all
   locally cached tarballs and repositories to be considered
   up-to-date.

src/libexpr/eval-settings.hh

@@ -185,7 +185,11 @@ struct EvalSettings : Config
         )"};
 
     Setting<bool> useEvalCache{this, true, "eval-cache",
-        "Whether to use the flake evaluation cache."};
+        R"(
+            Whether to use the flake evaluation cache.
+            Certain commands won't have to evaluate when invoked for the second time with a particular version of a flake.
+            Intermediate results are not cached.
+        )"};
 
     Setting<bool> ignoreExceptionsDuringTry{this, false, "ignore-try",
         R"(