docs(api): add description for coerce behavior for array type argument

[yxliang01]

This fixes #1389 . The wording I feel is not so ideal. Maybe suggestions needed.

[mleguen]

I agree with you about the wording.

I am afraid it would lead to a confusion between calling coerce for one key accepting an array of arguments, and calling coerce with an array of key (which is what I understood first reading your change).

I would suggest rephrasing it this way:

The coercion function should accept one argument, representing the parsed value from the command line (an array if multiple values are parsed for the key), and should return a new value or throw an error. The returned value will be used as the value for key (or one of its aliases) in argv.

[bcoe]

👍 I like @mleguen's edit; mind making the update @yxliang01?

[yxliang01]

Thanks @mleguen @bcoe for your suggestions! I also like @mleguen 's new wording. The changes have been made. :)

[mleguen]

Thanks @yxliang01

[mleguen]

The CI failing is weird, as it is coming from tests unrelated with any changes in this PR?

[mleguen]

The CI is failing on tests which are neither in the branch to merge, nor in the master branch.

I find these failing tests in a commit from @laggingreflex, in its fork, for which I can see a CI build failure in Travis.

@bcoe Is something wrong with Travis?

[yxliang01]

@mleguen Yep. This only changes documentation - shouldn't affect CI result at all. I think for this PR, CI can be ignored? :)