Adding 'attr' option to the Textarea options list

[ronanguilloux]

Q	A
Doc fix?	yes
New docs?	nope
Applies to	all?
Fixed tickets	?

attr is a quite useful option, but not yet listed here, so I added it.
BTW, the attr.rst.inc file already includes an attr conf. example concerning textareas.

[weaverryan]

Hey @ronanguilloux!

Hmm, you're right! This is one of the more important options, and we're actually missing it on every field (in a few cases, like "collection", that's ok - I don't think attr makes sense for it).

I think we should add this option to ever field that it makes sense to. That's a bit of work. Would you like to do it? If not, no worries - I'll merge this PR and we'll add it in another.

Thanks!

[weaverryan]

Also, if you do add it to the other fields, we'll need this PR to be rebased onto the 2.3 branch (else we'll likely get merge conflicts trying to merge this into 2.3): #4134 (comment)

Thanks!

[stof]

it makes sense even for the collection type too

[xabbuh]

This is related to #2360.

[weaverryan]

I'll leave adding this to all the other types to another PR (I mentioned it explicitly in #2360). Thanks Ronan!