tools lib traceevent: Man pages for event fields APIs

Create man pages for libtraceevent APIs:

  tep_event_common_fields(),
  tep_event_fields()
Signed-off-by: Tzvetomir Stoyanov <tstoyanov@vmware.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linux-trace-devel@vger.kernel.org
Link: http://lore.kernel.org/linux-trace-devel/20190503091119.23399-22-tstoyanov@vmware.com
Link: http://lkml.kernel.org/r/20190510200109.421670142@goodmis.orgSigned-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/traceevent/Documentation/libtraceevent-fields.txt

+libtraceevent(3)
+================
+
+NAME
+----
+tep_event_common_fields, tep_event_fields - Get a list of fields for an event.
+
+SYNOPSIS
+--------
+[verse]
+--
+*#include <event-parse.h>*
+
+struct tep_format_field pass:[*]pass:[*]*tep_event_common_fields*(struct tep_event pass:[*]_event_);
+struct tep_format_field pass:[*]pass:[*]*tep_event_fields*(struct tep_event pass:[*]_event_);
+--
+
+DESCRIPTION
+-----------
+The _tep_event_common_fields()_ function returns an array of pointers to common
+fields for the _event_. The array is allocated in the function and must be freed
+by free(). The last element of the array is NULL.
+
+The _tep_event_fields()_ function returns an array of pointers to event specific
+fields for the _event_. The array is allocated in the function and must be freed
+by free(). The last element of the array is NULL.
+
+RETURN VALUE
+------------
+Both _tep_event_common_fields()_ and _tep_event_fields()_ functions return
+an array of pointers to tep_format_field structures in case of success, or
+NULL in case of an error.
+
+EXAMPLE
+-------
+[source,c]
+--
+#include <event-parse.h>
+...
+struct tep_handle *tep = tep_alloc();
+...
+int i;
+struct tep_format_field **fields;
+struct tep_event *event = tep_find_event_by_name(tep, "kvm", "kvm_exit");
+if (event != NULL) {
+	fields = tep_event_common_fields(event);
+	if (fields != NULL) {
+		i = 0;
+		while (fields[i]) {
+			/*
+			  walk through the list of the common fields
+			  of the kvm_exit event
+			*/
+			i++;
+		}
+		free(fields);
+	}
+	fields = tep_event_fields(event);
+	if (fields != NULL) {
+		i = 0;
+		while (fields[i]) {
+			/*
+			  walk through the list of the event specific
+			  fields of the kvm_exit event
+			*/
+			i++;
+		}
+		free(fields);
+	}
+}
+...
+--
+
+FILES
+-----
+[verse]
+--
+*event-parse.h*
+	Header file to include in order to have access to the library APIs.
+*-ltraceevent*
+	Linker switch to add when building a program that uses the library.
+--
+
+SEE ALSO
+--------
+_libtraceevent(3)_, _trace-cmd(1)_
+
+AUTHOR
+------
+[verse]
+--
+*Steven Rostedt* <rostedt@goodmis.org>, author of *libtraceevent*.
+*Tzvetomir Stoyanov* <tz.stoyanov@gmail.com>, author of this man page.
+--
+REPORTING BUGS
+--------------
+Report bugs to  <linux-trace-devel@vger.kernel.org>
+
+LICENSE
+-------
+libtraceevent is Free Software licensed under the GNU LGPL 2.1
+
+RESOURCES
+---------
+https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git