docs(guide/filter): imrpove explanation of "pure function"

Improve the explanation of what a "pure function" is in simple words. The previous explanation
could be confusing, especially since the term "idempotent" (here used in it's broader
"Computer Science" meaning) is overloaded and has much stricter semantics in Mathematics or pure
Functional Programming.

Closes #15173

Author: 3noch

docs/content/guide/filter.ngdoc

@@ -109,8 +109,9 @@ as the first argument. Any filter arguments are passed in as additional argument
 function.
 
 The filter function should be a [pure function](http://en.wikipedia.org/wiki/Pure_function), which
-means that it should be stateless and idempotent, and not rely for example on other Angular services.
-Angular relies on this contract and will by default execute a filter only when the inputs to the function change.
+means that it should always return the same result given the same input arguments and should not affect
+external state, for example, other Angular services. Angular relies on this contract and will by default
+execute a filter only when the inputs to the function change.
 {@link guide/filter#stateful-filters Stateful filters} are possible, but less performant.
 
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