doc: adds 'close' events to fs.ReadStream and fs.WriteStream

Add 'close' event to doc/api/fs.md --> fs.ReadStream
Add 'close' event to doc/api/fs.md --> fs.WriteStream
Add 'close event to doc/api/stream.md --> stream.Writable

From squashed history:
Add 'close' event to stream.Writable per Issue #6484
Add #### prefix to Event: 'close' and backticks to 'close'
similar to stream.Readable event: 'close' section
Add more specifics to 'close' events for fs.ReadStream
and fs.WriteStream
Fix/Changed 'close' event from 'fs.ReadStream' to 'fs.WriteStream'
wrapped long lines at 80 chars, reworded
per Issue #6484
including the 'close' event as optional
add 'close' event as optional in stream.Readable
per issue #6484
doc: Add 'close' events to fs.ReadStream, 80char nit

Fixes: #6484
PR-URL: https://github.com/nodejs/node/pull/6499


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>