Add trendline documentation

Signed-off-by: James Duong <[email protected]>

docs/user/ppl/cmd/trendline.rst

@@ -0,0 +1,63 @@
+=============
+rename
+=============
+
+.. rubric:: Table of contents
+
+.. contents::
+   :local:
+   :depth: 2
+
+
+Description
+============
+| Use the ``trendline`` command to calculate the moving average on one or more fields in a search result.
+
+
+Syntax
+============
+trendline <average-type>(<number-of-samples>, <source-field>) AS <target-field> [" " <average-type>(<number-of-samples>, <source-field>) AS <target-field> ]...
+
+* average-type: mandatory. The moving average computation. Can be ``sma`` (simple moving average) currently.
+* number-of-samples: mandatory. The number of samples to use in the average calculation. Must be a positive non-zero integer.
+* source-field: mandatory. The field to compute the average on.
+* target-field: mandatory. The field name to report the computation under.
+
+
+Example 1: Calculate the moving average on one field.
+=====================================================
+
+The example shows how to calculate the moving average on one field.
+
+PPL query::
+
+    os> source=accounts | trendline sma(2, account_number) as an | fields an;
+    fetched rows / total rows = 4/4
+    +------+
+    | an   |
+    |------|
+    | null |
+    | 3.5  |
+    | 9.5  |
+    | 15.5 |
+    +------+
+
+
+Example 2: Calculate the moving average on multiple fields.
+===========================================================
+
+The example shows how to calculate the moving average on multiple fields.
+
+PPL query::
+
+    os> source=accounts | trendline sma(2, account_number) as an sma(2, age) as age_trend | fields an, age_trend ;
+    fetched rows / total rows = 4/4
+    +------+-----------+
+    | an   | age_trend |
+    |------|-----------|
+    | null | null      |
+    | 3.5  | 34.0      |
+    | 9.5  | 32.0      |
+    | 15.5 | 30.5      |
+    +------+-----------+
+