encoding: Add missing documentation for all public base32 functions

[nahuakang]

Add missing documentation per v missdoc command for #7047:

$ v missdoc vlib/encoding/                                
/home/nahua/projects/v/vlib/encoding/base32/base32.v:27:0:pub fn decode_string_to_string(src string) ?string 
/home/nahua/projects/v/vlib/encoding/base32/base32.v:31:0:pub fn decode_to_string(src []u8) ?string 
/home/nahua/projects/v/vlib/encoding/base32/base32.v:36:0:pub fn decode(src []u8) ?[]u8 
/home/nahua/projects/v/vlib/encoding/base32/base32.v:41:0:pub fn encode_string_to_string(src string) string 
/home/nahua/projects/v/vlib/encoding/base32/base32.v:45:0:pub fn encode_to_string(src []u8) string 
/home/nahua/projects/v/vlib/encoding/base32/base32.v:49:0:pub fn encode(src []u8) []u8 
/home/nahua/projects/v/vlib/encoding/base32/base32.v:54:0:pub fn (enc &Encoding) encode_to_string(src []u8) string 
/home/nahua/projects/v/vlib/encoding/base32/base32.v:58:0:pub fn (enc &Encoding) encode_string_to_string(src string) string 
/home/nahua/projects/v/vlib/encoding/base32/base32.v:62:0:pub fn new_std_encoding() Encoding 
/home/nahua/projects/v/vlib/encoding/base32/base32.v:66:0:pub fn new_std_encoding_with_padding(padding u8) Encoding 
/home/nahua/projects/v/vlib/encoding/base32/base32.v:70:0:pub fn new_encoding(alphabet []u8) Encoding 
/home/nahua/projects/v/vlib/encoding/base32/base32.v:74:0:pub fn new_encoding_with_padding(alphabet []u8, padding_char u8) Encoding 
/home/nahua/projects/v/vlib/encoding/base32/base32.v:190:0:pub fn (enc &Encoding) decode_string(src string) ?[]u8 
/home/nahua/projects/v/vlib/encoding/base32/base32.v:199:0:pub fn (enc &Encoding) decode_string_to_string(src string) ?string 
/home/nahua/projects/v/vlib/encoding/base32/base32.v:204:0:pub fn (enc &Encoding) decode(src []u8) ?[]u8 

The following could be updated to #7047:

• vlib/encoding/base32/base32.v <- Add this as checked
• vlib/encoding/binary/binary.v <- Remove this checkbox
• vlib/encoding/csv/reader.v <- Remove this checkbox
• vlib/encoding/csv/writer.v <- Remove this checkbox
• vlib/encoding/utf8/utf8.v <- Remove this checkbox