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.

Ref: nodejs#7868

doc/api/fs.md

@@ -1827,7 +1827,8 @@ added: v0.0.2
 
 Write `buffer` to the file specified by `fd`.
 
-`offset` and `length` determine the part of the buffer to be written.
+`offset` determines the part of the buffer to be written, and `length` is
+an integer specifying the number of bytes to write.
 
 `position` refers to the offset from the beginning of the file where this data
 should be written. If `typeof position !== 'number'`, the data will be written
@@ -2217,4 +2218,4 @@ The following constants are meant for use with the [`fs.Stats`][] object's
 [`event ports`]: http://illumos.org/man/port_create
 [`ReadDirectoryChangesW`]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365465%28v=vs.85%29.aspx
 [`AHAFS`]: https://www.ibm.com/developerworks/aix/library/au-aix_event_infrastructure/
-[Common System Errors]: errors.html#errors_common_system_errors
+[Common System Errors]: errors.html#errors_common_system_errors