;update CLI usage texts

hledger/Hledger/Cli/Commands/Check.txt

@@ -1,4 +1,47 @@
 check
-Check for various kinds of issue in the data.
+Check for various kinds of errors in your data.
 
 _FLAGS
+
+hledger provides a number of built-in error checks to help prevent
+problems in your data. Some, but not all, of these are run automatically
+before all commands. You can also use this check command to run any of
+the available tests. They are named, and run, as follows:
+
+hledger check runs the basic checks, like all other commands, but with
+no output unless there is a problem. These are:
+
+-   parseable - data files are well-formed and can be successfully
+    parsed
+-   autobalanced - all transactions are balanced, inferring missing
+    amounts where necessary, and possibly converting commodities using
+    transaction prices or automatically-inferred transaction prices
+-   assertions - all balance assertions are passing (except with
+    -I/--ignore-assertions)
+
+hledger check --strict also runs the additional "strict mode" checks,
+which are:
+
+-   accounts - all account names used by transactions have been declared
+-   commodities - all commodity symbols used have been declared
+
+hledger check CHECK1 CHECK2 ... runs all of the named checks, in turn.
+This may be useful when neither the default nor strict checks are
+exactly what you want, or when you want to focus on a single check of
+interest. The arguments are standard lowercase names for the checks.
+Currently only these checks can be run in this way:
+
+-   dates - transactions are ordered by date (similar to the old
+    check-dates command)
+-   leafnames - all account leaf names are unique ((similar to the old
+    check-dupes command)
+
+See also:
+
+Some checks are shipped as addon scripts for now (cf
+https://github.com/simonmichael/hledger/tree/master/bin, and Cookbook ->
+Scripting):
+
+-   tagfiles - all tag values containing / (a forward slash) exist as
+    file paths
+-   fancyassertions - more complex balance assertions are passing