Watcher Docs: Edited chain input topic & added chain to customizing watches.

Original commit: elastic/x-pack-elasticsearch@9e1d7f09c6

watcher/docs/customizing-watches.asciidoc

@@ -9,8 +9,8 @@ you can customize a watch by modifying its <<changing-inputs, inputs>>,
 
 [[changing-inputs]]
 === Changing Inputs
-Watcher supports three types of inputs <<loading-static-data, simple>>,
+Watcher supports four types of inputs <<loading-static-data, simple>>,
-<<loading-search-results, search>>, and <<loading-http-data, http>>.
+<<loading-search-results, search>>, <<loading-http-data, http>>, and <<input-chain, chain>>.
 
 [[loading-static-data]]
 ==== Loading Static Data with the Simple Input
@@ -115,6 +115,13 @@ during the month of May, 2015 that were tagged with `elasticsearch`.
     included as part of the path.
 ////////////    
 
+[[chaining-inputs]]
+==== Chaining Inputs
+
+You can create an <<input-chain, input chain>> to load data from multiple sources into a watch. 
+The inputs in a chain are processed in order, so the the data loaded by one input can be used 
+by subsequent inputs.
+
 [[changing-conditions]]
 === Changing Conditions
 

watcher/docs/reference/input/chain.asciidoc

@@ -1,18 +1,22 @@
 [[input-chain]]
 ==== Chain Input
 
-An <<input, input>> that enables you to chain several inputs one after the other and have each input populating the execution context.
+An <<input, input>> that enables you to string together multiple inputs to load data into the watch execution
-The `chain` input is useful when the output of one input should change the input of another or you need several inputs to gather all the
+context. Each input loads data into the execution context and that data is available to subsequent inputs in the chain.
-information needed for an action..
 
-You can define the chained input as following
+The `chain` input is useful when you want to perform an action based on data from multiple sources. 
+You can also use the data collected by one input to load data from another source. 
+
+To reference data loaded by a particular input, you use the input's name, `ctx.payload.<input-name>.<value>`.
+
+For example, the following chain input loads data from an HTTP server using the path set by a `simple` input.
 
 [source,json]
 --------------------------------------------------
 {
   "input" : {
     "chain" : {
-      "inputs" : [
+      "inputs" : [ <1>
         "first" : {
           "simple" : { "path" : "/_search" }
         },
@@ -21,7 +25,7 @@ You can define the chained input as following
             "request" : {
               "host" : "localhost",
               "port" : 9200,
-              "path" : "{{ctx.payload.first.path}}"
+              "path" : "{{ctx.payload.first.path}}" <2>
             }
           }
         }
@@ -31,9 +35,5 @@ You can define the chained input as following
   ...
 }
 --------------------------------------------------
-
+<1> The inputs in a chain are specified as an array to guarantee the order in which the inputs are processed. (JSON does not guarantee the order of arbitrary objects.)
-As you can see, the name of the input (`first` and `second` in this example) can be used to access values from the context in consecutive inputs.
+<2> Loads the `path` set by the `first` input.
-
-In case you are wondering about the structure of this input. The `inputs` must be an array, because JSON does not guarantee the order of arbitrary
-objects, one has to use a list.
-