Improve documentation on verify_pattern

[pboddupalli]

Description of the new feature

The main documentation page of fio on the use of verify_pattern is unclear. For example, the documentation reads:

"Or use combination of everything:
verify_pattern=0xff%o"abcd"-12"

The first 0xff seems to refer to the pattern that one wants to write. The next %o refers to

"Also, verify_pattern supports %o format, which means that for each block offset will be written and then verified back"

But what does the remaining "abcd"-12 refer to ?

[vincentkfu]

This is just the concatenation of the four different expressions: 0xff, the offset, the string "abcd", and the value -12. If you have a suggestion for improving the documentation, feel free to submit a pull request.