added details on what derivations are, added/removed periods (#1074)

* added details on what derivations are, added/removed periods

when i was reading the tutorial and came across impurities, it mentioned derivations and i had totally forgotten what they were. looking back, they were mentioned once in the very beginning and lightly discusses after. i'd expect others to be like me and would appreciate a reminder.

source/tutorials/nix-language.md

@@ -1740,18 +1740,19 @@ For historical reasons, some of the functions in `pkgs.lib` are equivalent to [`
 So far we have only covered what we call *pure expressions*:
 declaring data and transforming it with functions.
 
-In practice, describing derivations requires observing the outside world.
+In practice, describing derivations – the Nix language's defining feature, which enables functional programming with the file system – requires observing the outside world.
+We will discuss [derivations](derivations) later in the tutorial.
 
 There is only one impurity in the Nix language that is relevant here:
-reading files from the file system as *build inputs*
+reading files from the file system as *build inputs*.
 
 Build inputs are files that derivations refer to in order to describe how to derive new files.
 When run, a derivation will only have access to explicitly declared build inputs.
 
 The only way to specify build inputs in the Nix language is explicitly with:
 
 - File system paths
-- Dedicated functions.
+- Dedicated functions
 
 Nix and the Nix language refer to files by their content hash. If file contents are not known in advance, it's unavoidable to read files during expression evaluation.