Also:
* Clarify that padding refers to ‘output’ padding
* Make CalculateBase64EscapedLen() a bit more discoverable
* Reference all relevant non-obsolete RFCs (2045 as well as 4648)
PiperOrigin-RevId: 489981278
Change-Id: I27365ea5d52bfdb0c9d02ed96d05b4a60f2dc36f
