docs: add warning to per_job_logs option

Add a warning about the file open mode used when per_job_logs is set to
false. Log files are opened in append mode, so if the files already
exist, the previous contents will not be overwritten.

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

HOWTO.rst

@@ -3968,9 +3968,11 @@ Measurements and reporting
 
 .. option:: per_job_logs=bool
 
- If set, this generates bw/clat/iops log with per file private filenames. If
- not set, jobs with identical names will share the log filename. Default:
- true.
+ If set to true, fio generates bw/clat/iops logs with per job unique
+ filenames. If set to false, jobs with identical names will share a log
+ filename. Note that when this option is set to false log files will be
+ opened in append mode and if log files already exist the previous
+ contents will not be overwritten. Default: true.
 
 .. option:: group_reporting
 

fio.1

@@ -3667,8 +3667,10 @@ interpreted in seconds.
 .SS "Measurements and reporting"
 .TP
 .BI per_job_logs \fR=\fPbool
-If set, this generates bw/clat/iops log with per file private filenames. If
-not set, jobs with identical names will share the log filename. Default:
+If set to true, fio generates bw/clat/iops logs with per job unique filenames.
+If set to false, jobs with identical names will share a log filename. Note that
+when this option is set to false log files will be opened in append mode and if
+log files already exist the previous contents will not be overwritten. Default:
 true.
 .TP
 .BI group_reporting