Document different gcode data block encoding format in specification

[mtreinish]

Right now the G-Code data block just lists the encoding header format and lists the possible values for the header field:

0 = No encoding
1 = MeatPack algorithm
2 = MeatPack algorithm modified to keep comment lines

without defining what MeatPack or the modified MeatPack algorithms are. It would be good if the definition of those encoding types were documented (also assuming no encoding just means the text gcode). While you can look at the source code, documenting the actual specifics of the encoding formats will make the specification more useful for people trying to interoperate with the format.