Documentation: Update ftrace-design.txt

Correct grammos.  Spell out words.  Add missing words.
Consistent use of "mcount()" function name.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <4B2C0D78.6060707@oracle.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>