encoding/base64: documentation can show the alphabet used for encoding

[shaneHowearth]

The encoding variables make reference to the RFC on how they work, but it's not clear from Go's documentation, nor the RFC, how they differ (note that they are defined in the RFC, but the naming, and link, means it's not an easy lookup for the first time users, I broke some code at work not knowing the difference and it was discovered because a consumer was using the other set) - is there a cleaner way to say
URLEncoding uses the following characterset

 Table 2: The "URL and Filename safe" Base 64 Alphabet

     Value Encoding  Value Encoding  Value Encoding  Value Encoding
         0 A            17 R            34 i            51 z
         1 B            18 S            35 j            52 0
         2 C            19 T            36 k            53 1
         3 D            20 U            37 l            54 2
         4 E            21 V            38 m            55 3
         5 F            22 W            39 n            56 4
         6 G            23 X            40 o            57 5
         7 H            24 Y            41 p            58 6
         8 I            25 Z            42 q            59 7
         9 J            26 a            43 r            60 8
        10 K            27 b            44 s            61 9
        11 L            28 c            45 t            62 - (minus)
        12 M            29 d            46 u            63 _
        13 N            30 e            47 v           (underline)
        14 O            31 f            48 w
        15 P            32 g            49 x
        16 Q            33 h            50 y         (pad) =

and StdEncoding uses the following

                      Table 1: The Base 64 Alphabet

     Value Encoding  Value Encoding  Value Encoding  Value Encoding
         0 A            17 R            34 i            51 z
         1 B            18 S            35 j            52 0
         2 C            19 T            36 k            53 1
         3 D            20 U            37 l            54 2
         4 E            21 V            38 m            55 3
         5 F            22 W            39 n            56 4
         6 G            23 X            40 o            57 5
         7 H            24 Y            41 p            58 6
         8 I            25 Z            42 q            59 7
         9 J            26 a            43 r            60 8
        10 K            27 b            44 s            61 9
        11 L            28 c            45 t            62 +
        12 M            29 d            46 u            63 /
        13 N            30 e            47 v
        14 O            31 f            48 w         (pad) =
        15 P            32 g            49 x
        16 Q            33 h            50 y

As well as the reasoning that is already present in the docs.

[cherrymui]

The encoding is standard and not defined by Go. I think reference the RFC is the right approach.

cc @rsc

[shaneHowearth]

FTR - I'm not suggesting that Go is defining the character sets, I'm asking that the character sets being used be explicitly mentioned in the documentation (As I say, a bug occurred due to a misunderstanding of the character sets, with one tool claiming that the set used meant our implementation was "broken" - when all it was was URL Encoding had been used instead of standard).

Also - the links drop you at the index of the RFC, which isn't that flash, meaning that people have to work out that they were dropped at the index, not at the point that the character sets for their chosen encoding is defined

[gopherbot]

Change https://go.dev/cl/525296 mentions this issue: encoding: show the alphabet for base32 and base64