encoding/binary: varint functions could be better described

[agl]

The comments for the varint functions in encoding/binary don't mention anything about how the integer is encoded. Since there are quite a number of options, it should probably be more clear that it's a little-endian, base128 format.

[cznic]

Package documentation says:

The varint functions encode and decode single integer values using a variable-length encoding; smaller values require fewer bytes. For a specification, see https://developers.google.com/protocol-buffers/docs/encoding.

[griesemer]

@cznic Thanks for the hint. I agree, that seems good enough (we don't want to repeat everything for each of the 4 functions, I assume).