Should clarify expected handling of unused bits for float and double value encoding

[cowtowncoder]

As per:

FasterXML/jackson-dataformats-binary#301

it looks like Smile Java doc by Jackson (jackson-dataformats-binary) has somewhat inconsistent encoding behavior of unused bits. It would make sense to clarify expected handling, along the lines of (I think?):

1. Contents of unused bits MUST be ignored by decoder
2. Unused bits SHOULD be left as zeroes by encoder

[cowtowncoder]

Updated

https://github.com/FasterXML/smile-format-specification/blob/master/smile-specification.md

but no version change (since behavior clarified, not changed).

Will need to tackle Jackson Smile codec in future -- likely for 2.14 as this may (even it should not) affect some decoders, issue:

FasterXML/jackson-dataformats-binary#300