doc: clarifying variables in fs.write()

This commit clarifies variables in the Filesystem docs.
Prior, the documentation for fs.write() had an ambiguous
remark on the parameters of offset and length.

Therefore, this commit makes explicit that the length parameter
in fs.write() is used to denote the number of bytes, which is
a clearer reference for its usage.

PR-URL: https://github.com/nodejs/node/pull/9792
Ref: https://github.com/nodejs/node/issues/7868


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>