docs: ftrace: clarify when tracing is disabled by the trace file

The current text could mislead the user into believing that only read()
disables tracing. Clarify that any open() call that requests read access
disables tracing.

Link: https://lkml.kernel.org/r/CAADnVQ+hU6QOC_dPmpjnuv=9g4SQEeaMEMqXOS2WpMj=q=LdiQ@mail.gmail.com
Signed-off-by: Peter Wu <peter@lekensteyn.nl>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/trace/ftrace.rst

@@ -125,7 +125,8 @@ of ftrace. Here is a list of some of the key files:
 
 	This file holds the output of the trace in a human
 	readable format (described below). Note, tracing is temporarily
-	disabled while this file is being read (opened).
+	disabled when the file is open for reading. Once all readers
+	are closed, tracing is re-enabled.
 
   trace_pipe:
 
@@ -139,8 +140,9 @@ of ftrace. Here is a list of some of the key files:
 	will not be read again with a sequential read. The
 	"trace" file is static, and if the tracer is not
 	adding more data, it will display the same
-	information every time it is read. This file will not
-	disable tracing while being read.
+	information every time it is read. Unlike the
+	"trace" file, opening this file for reading will not
+	temporarily disable tracing.
 
   trace_options:
 
@@ -3153,7 +3155,10 @@ different. The trace is live.
 
 
 Note, reading the trace_pipe file will block until more input is
-added.
+added. This is contrary to the trace file. If any process opened
+the trace file for reading, it will actually disable tracing and
+prevent new entries from being added. The trace_pipe file does
+not have this limitation.
 
 trace entries
 -------------