tracing: Amend documentation in recordmcount.pl to reflect implementation

The documentation currently says we will use the first function in a section
as a reference. The actual algorithm is: choose the first global function we
meet as a reference. If there is none, choose the first local one.
Change the documentation to be consistent with the code.

Also add several other clarifications.

Signed-off-by: Li Hong <lihong.hi@gmail.com>
LKML-Reference: <20091028050138.GA30758@uhli>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>