Wrong description for flag usage

[yohann-bacha]

So my idea for the CLI was to specify the optional flags in the usage text.
For example, for the alerts-add command we have the following usage :

NAME:
   scalingo alerts-add - Add an alert to an application

USAGE:
   scalingo alerts-add [command options] [arguments...]
....

As you see, the usage isn't explicit at all.
The command actually have 3 required flags, and 4 optional flags.
Actually, nothing specifies the optional flags expect in the description :

CATEGORY:
   Alerts

DESCRIPTION:
   Add an alert to an application metric.

      The "duration-before-trigger", "remind-every", "below" and "notifiers" flags are optionnal

      Example
...

But this should not be
Technically, following the POSIX Utility Conventions, the usage should contains the flags and arguments that :

• Are optional are surrounded by brackets [--optional-flag]
• Are required aren't surrounded by anything

The usage for this command should be :

scalingo alerts-add --container-type value --metric value --limit value [--duration-before-trigger value] 

Or the short version :

scalingo alerts-add -c value -m value -l value [--duration-before-trigger value] [-r value] [-b value] [-n value]

On the paper this is simple. BUT the library we use (urfave/cli) doesn't permit us to customize the usage that much. I struggled a lot reading the source code of the library, thinking about how I could do, to resign that it wasn't possible.
We can provide a CustomUsageTemplate, but in order to correctly fill the usage, we need more information that would need a pull request to urfave/cli to change the library's code.

[guillaume-sc]

@yohann-bacha,i s the above still valid, after what you changed to the CLI help?