docs: unclear if/when there's a minimum tilt version needed

[lizzthabet]

it's not always clear in our documentation if a user needs a minimum version of tilt to use certain api functions and arguments. if there is a certain version of tilt needed, but a user hasn't upgraded, they'll get a generic message that says something like:

Error: local_resource: unexpected keyword argument "dir"

even though the documentation says this argument exists. this could be pretty confusing or frustrating if you haven't used tilt a bunch.

• one example: the dir argument in local_resource is only supported in v0.20.8+
• another example: the labels argument in *_resource functions is only supported in v0.22.1+. (if a user does a lot of clicking around, they may see the resource grouping blog post and see the version note there.)

solutions for this could be:

• add better messaging for tiltfile error logs, encouraging a user to check for typos or update their tilt version
• add annotations or notes to the tilt api docs, saying something like "supported in v0.XX.x+"