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: #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>

Author: Jenna Vuong

doc/api/fs.md

@@ -151,6 +151,11 @@ Stop watching for changes on the given `fs.FSWatcher`.
 
 Emitted when the ReadStream's file is opened.
 
+### Event: 'close'
+
+Emitted when the `ReadStream`'s underlying file descriptor has been closed
+using the `fs.close()` method.
+
 ### readStream.path
 
 The path to the file the stream is reading from as specified in the first
@@ -238,6 +243,11 @@ on Unix systems, it never was.
 
 Emitted when the WriteStream's file is opened.
 
+### Event: 'close'
+
+Emitted when the `WriteStream`'s underlying file descriptor has been closed
+using the `fs.close()` method.
+
 ### writeStream.bytesWritten
 
 The number of bytes written so far. Does not include data that is still queued

doc/api/stream.md

@@ -160,7 +160,8 @@ Emitted when the stream and any of its underlying resources (a file
 descriptor, for example) have been closed. The event indicates that
 no more events will be emitted, and no further computation will occur.
 
-Not all streams will emit the `'close'` event.
+Not all streams will emit the `'close'` event as the `'close'` event is
+optional.
 
 #### Event: 'data'
 
@@ -573,6 +574,15 @@ Examples of writable streams include:
 * [child process stdin][]
 * [`process.stdout`][], [`process.stderr`][]
 
+#### Event: 'close'
+
+Emitted when the stream and any of its underlying resources (a file descriptor,
+for example) have been closed. The event indicates that no more events will be
+emitted, and no further computation will occur.
+
+Not all streams will emit the `'close'` event as the `'close'` event is
+optional.
+
 #### Event: 'drain'
 
 If a [`stream.write(chunk)`][stream-write] call returns `false`, then the