make introduction shorter

doc/manual/src/language/import-from-derivation.md

@@ -1,14 +1,14 @@
 # Import From Derivation
 
-Import From Derivation (IFD) is a Nix language pattern where an expression's value depends on the contents of a [store object] produced by a [derivation].
-It is characterised by the fact that the store object has to be [realise]d before the expression can be evaluated.
+The value of a Nix expression can depend on the contents of a [store object] produced by a [derivation].
+In this case, when that store object is needed, evaluation will be paused, the store object [realise]d, and then evaluation resumed.
 
 [store object]: /glossary.md#gloss-store-object
 [derivation]: /glossary.md#gloss-derivation
 [realise]: /glossary.md#gloss-realise
 
 This has performance implications:
-Since evaluation is sequential, each required derivation will also be built sequentially.
+Since evaluation is sequential, each required store object that is not already in the store will also be realised sequentially.
 
 Passing a derivation `drv` to any built-in function that reads from the filesystem constitutes Import From Derivation: