fix(cli): improve description of --json to reflect behavior

When using `cdk synth -o` the templates emitted to the cloud assembly are always in JSON. This change improves the documentation for `--json`, which only applies when `-o` is not used.

Closes #2965

packages/aws-cdk/bin/cdk.ts

@@ -33,7 +33,7 @@ async function parseCommandLineArguments() {
     .option('trace', { type: 'boolean', desc: 'Print trace for stack warnings' })
     .option('strict', { type: 'boolean', desc: 'Do not construct stacks with warnings' })
     .option('ignore-errors', { type: 'boolean', default: false, desc: 'Ignores synthesis errors, which will likely produce an invalid output' })
-    .option('json', { type: 'boolean', alias: 'j', desc: 'Use JSON output instead of YAML', default: false })
+    .option('json', { type: 'boolean', alias: 'j', desc: 'Use JSON output instead of YAML when templates are printed to STDOUT', default: false })
     .option('verbose', { type: 'boolean', alias: 'v', desc: 'Show debug logs', default: false })
     .option('profile', { type: 'string', desc: 'Use the indicated AWS profile as the default environment', requiresArg: true })
     .option('proxy', { type: 'string', desc: 'Use the indicated proxy. Will read from HTTPS_PROXY environment variable if not specified.', requiresArg: true })