perf_counter tools: add help texts

Add Documentation/perf-stat.txt and Documentation/perf-top.txt.

The template that was used for it: Documentation/git-add.txt from Git.

Fix up small bugs to make these help texts show up both in the 'perf'
common-command summary output screen, and on the individual help screens.

Signed-off-by: Ingo Molnar <mingo@elte.hu>