Document expectations around uploadBlob endpoint

[devinivy]

We should document the expected usage of com.atproto.repo.uploadBlob. In particular, it's not clear how long uploaded blobs are available to be linked into a record before they expire. A case where this came-up was a client interested in implementing scheduled posts: they were not sure whether or not it would be feasible to upload a blob a day or longer ahead of posting a record that uses it.

Current TLDR on this is:

• On our service we keep blobs around long enough for someone who is manually performing the upload then record creation, e.g. writing curl requests by hand. So there is no intense rush to create the record as fast as possible after the upload, but it is expected that the blob will be used in rough proximity to when it's uploaded.
• From a compatibility standpoint, PDSes will differ in how long they keep unused blobs around before expiring them, and we should set a baseline expectation which is not overbearing for PDS operators to uphold.