Document Postgres URL Requirements

We've seen in some cases folks have Postgres password with special characters in them that require URL encoding.

Since we take in the URL in both FF config and the migrations job, its not appropriate for the script to parse the URL and then (re-)encode the password since it could potentially already be encoded properly.

Instead, we just need to make sure this is well documented.

Start by locating the documentation for Postgres URLs used by the FF configuration and migrations job. Document that passwords containing special characters must be URL-encoded, while already encoded values should be passed through unchanged; done when both usage paths state this requirement clearly.

Written by the indexing model from the issue text.