check: doc

hledger/Hledger/Cli/Commands/Check.md

@@ -1,4 +1,44 @@
 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](hledger.html#input-files)
+- **autobalanced** - all transactions are [balanced](journal.html#postings), 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`)
+
+[transaction prices]: journal.html#transaction-prices
+[balance assertions]: journal.html#balance-assertions
+[strict mode]: hledger.html#strict-mode
+
+`hledger check --strict` also runs the additional "strict mode" checks,
+which are:
+
+- **accounts** - all account names used by transactions [have been declared](journal.html#account-error-checking)
+- **commodities** - all commodity symbols used [have been declared](journal.html#commodity-error-checking)
+
+`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](scripting.html)):
+
+- **tagfiles** - all tag values containing / (a forward slash) exist as file paths
+- **fancyassertions** - more complex balance assertions are passing