makes 'ipfs files write' usage docs clearer

[hackergrrl]

addresses #2380 and #2380 (comment)

[RichardLitt]

Doesn't truncate start where offset starts if offset is there, though?

[hackergrrl]

@RichardLitt no, it wipes the whole file first:

stephen // ~ $ ipfs files read /myfs/file
!!!?
hello world
stephen // ~ $ echo '&' | ipfs files write /myfs/file --offset 4 --truncate
stephen // ~ $ ipfs files read /myfs/file
&

---

cc @whyrusleeping -- is this consistent with what's intended?

[whyrusleeping]

ipfs files write /myfs/file --offset 4 --truncate equates to opening a file with O_TRUNC and calling seek() to set the position of the file descriptor to 4. Doing so will sparsely expand the file (with zeros) up to the point where you start writing.

[whyrusleeping]

these doc changes LGTM