reword description of language properties

the list style is supposed to give more structure.
each property is explained as concisely as possible while trying not to
sound too fancy.

doc/manual/src/language/index.md

@@ -1,13 +1,33 @@
 # Nix Language
 
-The Nix language is a pure, lazy, functional language. Purity
-means that operations in the language don't have side-effects (for
-instance, there is no variable assignment). Laziness means that
-arguments to functions are evaluated only when they are needed.
-Functional means that functions are “normal” values that can be passed
-around and manipulated in interesting ways. The language is not a
-full-featured, general purpose language. Its main job is to describe
-packages, compositions of packages, and the variability within packages.
+The Nix language is
 
-This section presents the various features of the language.
+- *domain-specific*
+
+  It only exists for the Nix package manager:
+  to describe packages and configurations as well as their variants and compositions.
+  It is not intended for general purpose use.
+
+- *declarative*
+
+  There is no notion of executing sequential steps.
+  Dependencies between operations are established only through data.
+
+- *pure*
+
+  Values cannot change during computation.
+  Functions always produce the same output if their input does not change.
+
+- *functional*
+
+  Functions are like any other value.
+  Functions can be assigned to names, taken as arguments, or returned by functions.
+
+- *lazy*
+
+  Expressions are only evaluated when their value is needed.
+
+- *dynamically typed*
+
+  Type errors are only detected when expressions are evaluated.