Updating opa fmt --help text

Signed-off-by: Johan Fylling <[email protected]>

cmd/fmt.go

@@ -59,7 +59,7 @@ is provided - this tool will use stdin.
 The format of the output is not defined specifically; whatever this tool outputs
 is considered correct format (with the exception of bugs).
 
-If the '-w' option is supplied, the 'fmt' command with overwrite the source file
+If the '-w' option is supplied, the 'fmt' command will overwrite the source file
 instead of printing to stdout.
 
 If the '-d' option is supplied, the 'fmt' command will output a diff between the
@@ -70,7 +70,26 @@ that would change if formatted. The '-l' option will suppress any other output
 to stdout from the 'fmt' command.
 
 If the '--fail' option is supplied, the 'fmt' command will return a non zero exit
-code if a file would be reformatted.`,
+code if a file would be reformatted.
+
+The 'fmt' command can be run in several compatibility modes for consuming and outputting
+different Rego versions:
+
+* 'opa fmt':
+  * v1 Rego is formatted to v1
+  * 'rego.v1'/'future.keywords' imports are NOT removed
+  * 'rego.v1'/'future.keywords' imports are NOT added if missing
+  * v0 rego is rejected
+* 'opa fmt --v0-compatible':
+  * v0 Rego is formatted to v0
+  * v1 Rego is rejected
+* 'opa fmt --v0-v1':
+  * v0 Rego is formatted to be compatible with v0 AND v1
+  * v1 Rego is rejected
+* 'opa fmt --v0-v1 --v1-compatible':
+  * v1 Rego is formatted to be compatible with v0 AND v1
+  * v0 Rego is rejected
+`,
 	PreRunE: func(cmd *cobra.Command, _ []string) error {
 		return env.CmdFlags.CheckEnvironmentVariables(cmd)
 	},