cloud storage: Add documentation for AWS_SESSION_TOKEN parameter

[rolandcrosby]

In cockroachdb/cockroach#32455, we added support for a new URL parameter, AWS_SESSION_TOKEN, for the S3 cloud storage endpoint. This needs to be added to the tables of parameters in the backup/restore/import docs. The parameter is used for AWS session token-based authentication, as documented in this guide; I don't think we need to go into a ton of detail on what it's for, since users aren't going to need it or look for it unless their organization is already using this kind of AWS credentials.

[jseldess]

@lhirata, does this overlap with one of your roadmap or release note issues?

[lnhsingh]

@jseldess - related to #4196

[jseldess]

OK. Closing as duplicate then.