Document the command_chain option

doc/ale.txt

@@ -1005,15 +1005,40 @@ ale#linter#Define(filetype, linter)                       *ale#linter#Define()*
                          check, and will produce the lines of output given to
                          the `callback`.
 
-                         Either this or the `command_callback` argument must
-                         be provided.
-
   `command_callback`       A |String| or |Funcref| for a callback function
                          accepting a buffer number. A |String| should be
                          returned for a command to run. This can be used in
                          place of `command` when more complicated processing
                          is needed.
 
+  `command_chain`          A |List| of |Dictionary| items defining a series
+                         of commands to be run. At least one |Dictionary|
+                         should be provided. Each Dictionary must contain the
+                         key `callback`, defining a |String| or |Funcref| for
+                         a function returning a |String| for a command to run.
+
+                         The callback functions for each command after the
+                         first command in in the chain should accept two
+                         arguments `(buffer, output)`, a buffer number and a
+                         |List| of lines of output from the previous command
+                         in the chain.
+
+                         The first callback function in a chain accepts only
+                         a `(buffer)` argument, as there are no previous
+                         commands to run which return `output`.
+
+                         Commands in the chain will all use the
+                         `output_stream` value provided in the root
+                         |Dictionary|. Each command in the chain can also
+                         provide an `output_stream` key to override this value.
+                         See the `output_stream` description for more
+                         information.
+
+                         The Vim buffer being checked for linter will only
+                         be sent to the final command in the chain. Previous
+                         commands in the chain will receive no input from
+                         stdin.
+
   `output_stream`          A |String| for the output stream the lines of output
                          should be read from for the command which is run. The
                          accepted values are `'stdout'`, `'stderr'`, and
@@ -1023,6 +1048,12 @@ ale#linter#Define(filetype, linter)                       *ale#linter#Define()*
                          instead of stdout. The option `'both'` will read
                          from both stder and stdout at the same time.
 
+  Only one of `command`, `command_callback`, or `command_chain` should be
+  specified. `command_callback` is generally recommended when a command string
+  needs to be generated dynamically, or any global options are used.
+  `command_chain` is recommended where any system calls need to be made to
+  retrieve some kind of information before running the final command.
+
   Some programs for checking for errors are not capable of receiving input
   from stdin, as is required by ALE. To remedy this, a wrapper script is
   provided named in the variable |g:ale#util#stdin_wrapper|. This variable