Substitute usage information into the documentation

This will simplify keeping documentation in sync with the code.

docs/src/using-task-command.md

@@ -4,6 +4,13 @@ The main interface to your tasks is the `ta` command, which supports various sub
 Customizable [reports](./reports.md) are also available as subcommands, such as `next`.
 The command reads a [configuration file](./config-file.md) for its settings, including where to find the task database.
 And the `sync` subcommand [synchronizes tasks with a sync server](./task-sync.md).
-You can find a list of all subcommands, as well as the built-in reports, with `ta help`.
 
 > NOTE: the `task` interface does not precisely match that of TaskWarrior.
+
+## Subcommands
+
+The sections below describe each subcommand of the `ta` command.
+The syntax of `[filter]` is defined in [filters](./filters.md), and that of `[modification]` in [modifications](./modifications.md).
+You can also find a summary of all subcommands, as well as filters, built-in reports, and so on, with `ta help`.
+
+<!-- INSERT GENERATED DOCUMENTATION - subcommands -->