Update fmt.mdx (#31553)

* Update fmt.mdx

Fix insufficient explanation of "-check" option.

* Apply suggestions from code review

Co-authored-by: kmoe <5575356+kmoe@users.noreply.github.com>

Co-authored-by: kmoe <5575356+kmoe@users.noreply.github.com>

website/docs/cli/commands/fmt.mdx

@@ -55,6 +55,5 @@ The command-line flags are all optional. The following flags are available:
 * `-list=false` - Don't list the files containing formatting inconsistencies.
 * `-write=false` - Don't overwrite the input files. (This is implied by `-check` or when the input is STDIN.)
 * `-diff` - Display diffs of formatting changes
-* `-check` - Check if the input is formatted. Exit status will be 0 if
-  all input is properly formatted and non-zero otherwise.
+* `-check` - Check if the input is formatted. Exit status will be 0 if all input is properly formatted. If not, exit status will be non-zero and the command will output a list of filenames whose files are not properly formatted.
 * `-recursive` - Also process files in subdirectories. By default, only the given directory (or current directory) is processed.