Adds more documentation around start()/stop() Tracker methods

[weslleyspereira]

Closes #12.

[Deln0r]

Verified at 485d1d2 by running it rather than reading the diff.

The reproducer from #12 now writes: start() then stop() with no arguments gives the same 532-byte file as the context manager. On 3d6aa73 it gives nothing. Suite is 186 passed, 5 skipped.

On the new sentence about starting and stopping one tracker several times: with output= set, every start() appends another # timestamp header, and file_to_df then stops there with ValueError: time data '#' does not match format.

That is not new here, write_header has always appended, but the sentence sends users into it. Either drop the sentence, or let file_to_df skip comment lines.

[weslleyspereira]

Thanks for the find! It should be fixed now