add doc

Signed-off-by: xinyual <xinyual@amazon.com>

Author: xinyual

docs/user/ppl/cmd/appendpipe.rst

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+=========
+appendpipe
+=========
+
+.. rubric:: Table of contents
+
+.. contents::
+   :local:
+   :depth: 2
+
+
+Description
+============
+| Using ``appendpipe`` command to appends the result of the subpipeline to the search results. Unlike a subsearch, the subpipeline is not run first.The subpipeline is run when the search reaches the appendpipe command.
+The appendpipe command is used to append the output of transforming commands, such as chart, timechart, stats, and top.
+The command aligns columns with the same field names and types. For different column fields between the main search and sub-search, NULL values are filled in the respective rows.
+
+Version
+=======
+3.3.0
+
+Syntax
+============
+appendpipe [<subpipeline>]
+
+* subpipeline: mandatory. A list of commands that are applied to the search results from the commands that occur in the search before the ``appendpipe`` command.
+
+Example 1: Append rows from a total count to existing search result
+====================================================================================
+
+This example appends rows from "total by gender" to "sum by gender, state" with merged column of same field name and type.
+
+PPL query::
+
+    os> source=accounts | stats sum(age) as part by gender, state | sort -sum | head 5 | appendpipe [ stats sum(part) as total by gender ];
+    fetched rows / total rows = 6/6
+    +------+--------+-------+-------+
+    | part | gender | state | total |
+    |------+--------+-------+-------|
+    | 36   | M      | TN    | null  |
+    | 33   | M      | MD    | null  |
+    | 32   | M      | IL    | null  |
+    | 28   | F      | VA    | null  |
+    | null | F      | null  | 28    |
+    | null | M      | null  | 101   |
+    +------+--------+-------+-------+
+
+
+
+Example 2: Append rows with merged column names
+===============================================================
+
+This example appends rows from "count by gender" to "sum by gender, state".
+
+PPL query::
+
+    os> source=accounts | stats sum(age) as total by gender, state | sort -`sum(age)` | head 5 | appendpipe [ stats sum(total) as total by gender ];
+    fetched rows / total rows = 6/6
+    +----------+--------+-------+
+    | total    | gender | state |
+    |----------+--------+-------|
+    | 36       | M      | TN    |
+    | 33       | M      | MD    |
+    | 32       | M      | IL    |
+    | 28       | F      | VA    |
+    | 28       | F      | null  |
+    | 101      | M      | null  |
+    +----------+--------+-------+
+
+Example 3: Append rows with column type conflict
+=============================================
+
+This example shows how column type conflicts are handled when appending results. Same name columns with different types will generate two different columns in appended result.
+
+PPL query::
+
+    os> source=accounts | stats sum(age) as total by gender, state | sort -`sum(age)` | head 5 | appendpipe [ stats sum(total) as total by gender | eval state = 123 ];
+    fetched rows / total rows = 6/6
+    +------+--------+-------+--------+
+    | sum  | gender | state | state0 |
+    |------+--------+-------+--------|
+    | 36   | M      | TN    | null   |
+    | 33   | M      | MD    | null   |
+    | 32   | M      | IL    | null   |
+    | 28   | F      | VA    | null   |
+    | 28   | F      | null  | 123    |
+    | 101  | M      | null  | 123    |
+    +------+--------+-------+--------+
+