Despite being mentioned at Documentation/trace/ftrace.rst as a
rst file, this file was still a text one, with several issues.

Convert it to ReST and add it to the trace index:
- Mark the document title as such;
- Identify and indent the literal blocks;
- Use the proper markups for table.

Signed-off-by: Mauro Carvalho Chehab
Acked-by: Steven Rostedt (VMware)
Signed-off-by: Jonathan Corbet