Descrive generators, range() with by to manual

docs/content/3.manual/manual.yml

@@ -1697,6 +1697,52 @@ sections:
             input: '[10,2,5,3]'
             output: ['20']
 
+      - title: Recursion
+        body: |
+
+          As described above, `recurse` uses recursion, and any jq
+          function can be recursive.  Tail calls are optmized.
+
+      - title: Generators and iterators
+        body: |
+
+            Some jq operators and functions are actually generators in
+            that they can produce zero, one, or more values for each
+            input, just as one might expect in other programming
+            languages that have generators.  For example, `.[]`
+            generates all the values in its input (which must be an
+            array or an object), `range(0; 10)` generates the integers
+            between 0 and 10, and so on.
+
+            Even the comma operator is a generator, generating first the
+            values generated by the expression to the left of the comma,
+            then for each of those, the values generate by the
+            expression on the right of the comma.
+
+            The `empty` builtin is the generator that produces zero
+            outputs.  The `empty` builtin backtracks to the preceding
+            generator expression.
+
+            All jq functions can be generators just by using builtin
+            generators.  It is also possible to define new generators
+            using only recursion and the comma operator.  If the
+            recursive call(s) is(are) "in tail position" then the
+            generator will be efficient.  In the example below the
+            recursive call by `_range` to itself is in tail position.
+            The example shows off three advanced topics: tail recursion,
+            generator construction, and sub-functions.
+
+        examples:
+          - program: 'def range(init; upto; by):
+                    def _range:
+                        if (by > 0 and . < upto) or (by < 0 and . > upto)
+                        then ., ((.+by)|_range)
+                        else . end;
+                    if by == 0 then init else init|_range end |
+                    select(. < upto);
+                range(0; 10; 3)'
+            input: 'null'
+            output: ['0,3,6,9']
 
   - title: Assignment
     body: |