Fix up encoding definitions/references

[inexorabletash]

Following dd2ddc7ce8b081ef10d913fc71b862059d8c1a4f there's this text:

To encode a string, run UTF-8 encode on string.

This text is unnecessary, as places that need to encode invoke "UTF-8 encode" directly. We should either:

• Remove that line; or
• Define encode and then simplify the call sites from "result of UTF-8 encoding var" to "result of encoding var"

At the same time, we should consider if this definition is worthwhile:

To decode a value, run UTF-8 decode without BOM on value.

In theory we could remove that and make the UTF-8 decoding inline, but the "without BOM" part makes it wordier.

cc: @recvfrom

[domenic]

Chiming in from the sidelines, I think using the wordier definitions and removing your own spec's definitions of "encode" and "decode" would make things clearer for the reader.