patch 8.0.0105

Problem:    When using ch_read() with zero timeout, can't tell the difference
            between reading an empty line and nothing available.
Solution:   Add ch_canread().

runtime/doc/channel.txt

@@ -418,7 +418,11 @@ This uses the channel timeout.  To read without a timeout, just get any
 message that is available: >
 	let output = ch_read(channel, {'timeout': 0})
 When no message was available then the result is v:none for a JSON or JS mode
-channels, an empty string for a RAW or NL channel.
+channels, an empty string for a RAW or NL channel.  You can use |ch_canread()|
+to check if there is something to read.
+
+Note that when there is no callback message are dropped.  To avoid that add a
+close callback to the channel.
 
 To read all output from a RAW channel that is available: >
 	let output = ch_readraw(channel)
@@ -470,6 +474,11 @@ This depends on the system (on Unix this happens because closing the write end
 of a pipe causes the read end to get EOF).  To avoid this make the job sleep
 for a short while before it exits.
 
+Note that if the job exits before you read the output, the output may be lost.
+This depends on the system (on Unix this happens because closing the write end
+of a pipe causes the read end to get EOF).  To avoid this make the job sleep
+for a short while before it exits.
+
 The handler defined for "out_cb" will not receive stderr.  If you want to
 handle that separately, add an "err_cb" handler: >
     let job = job_start(command, {"out_cb": "MyHandler",