Document size limits on fields in the event payload

[jjbayer]

Currently, field attributes like this one define how large a data bag in the event payload can get. The limits applied to each field are relevant to our users.

Let's include these limits in the auto-generated public documentation, and link to it from docs.sentry.io and / or develop.sentry.dev such that it is easy to find.

Relates to https://github.com/getsentry/relay/issues/2109.