Fix minor issues in the documentation of libtime.

- When the timezone is UTC, the "zone" field of the RFC 822 format is
  "GMT" (or "UT"), not "UTC."
- Although the name of `rfc3999` refers to RFC 3999, the documentation
  of it refers only to ISO 8601. This commit adds a description of the
  relation between ISO 8601 and RFC 3999.