Add note to binary! about how chars append with /part.

[greggirwin]

From Gitter chat: https://gitter.im/red/bugs?at=5ec5643626d26710bba95dbf

If we think of it as "The value is first coerced to a type compatible with the series, then the part is taken." we get what we have now. Easy to understand. This is similar:

>> append/part {} 'a/b/c 2
== "a/"

Where the path is first formed, then part of that taken. Knowing this behavior, we can add a doc note that shows how to work around it if you want to append full chars to binaries. e.g.

>> append #{} copy/part "^(2190)XYZ" 1
== #{E28690}