docs: clarify write_iolog description

Note that the log file is opened in append mode to avoid confusion when
the same log file is used repeatedly.

Signed-off-by: Vincent Fu <vincent.fu@samsung.com>

HOWTO.rst

diff --git a/HOWTO.rst b/HOWTO.rst
index 470777e..104cce2 100644
--- a/HOWTO.rst
+++ b/HOWTO.rst
@@ -3049,7 +3049,8 @@
 
 	Write the issued I/O patterns to the specified file. See
 	:option:`read_iolog`.  Specify a separate file for each job, otherwise the
-	iologs will be interspersed and the file may be corrupt.
+        iologs will be interspersed and the file may be corrupt. This file will
+        be opened in append mode.
 
 .. option:: read_iolog=str
 

fio.1

diff --git a/fio.1 b/fio.1
index 948c01f..ce9bf3e 100644
--- a/fio.1
+++ b/fio.1
@@ -2793,7 +2793,8 @@
 .BI write_iolog \fR=\fPstr
 Write the issued I/O patterns to the specified file. See
 \fBread_iolog\fR. Specify a separate file for each job, otherwise the
-iologs will be interspersed and the file may be corrupt.
+iologs will be interspersed and the file may be corrupt. This file will be
+opened in append mode.
 .TP
 .BI read_iolog \fR=\fPstr
 Open an iolog with the specified filename and replay the I/O patterns it