Add save_all notation to webp

[RadixSeven]

I just tried to use the WEBP writer to save an animation and was unable to without save_all=true. But that parameter was not mentioned in the documentation. This correction adds it.

Fixes

• Add save_all as an option in the docs for WEBP

[radarhere]

Thanks for trying to address the problem.

I wouldn't say that save_all isn't mentioned though - it's mentioned in the line just before your addition -

When calling save() to write a WebP file, the following options are available when the save_all argument is present and true.

So combined with your change -

When calling save() to write a WebP file, the following options are available when the save_all argument is present and true.

save_all
If present and true, all frames of the image will be saved. If not, then only the first frame of a multiframe image will be saved.

It is then saying that save_all can be used when ``save_all` is present... I find this to be confusing. I've created #5016 as an alternative - let us know if you have any thoughts on that.

[RadixSeven]

I like #5016 better, so I'm closing this PR.