fix timing utility docstrings for tic toc tac

[mmcky]

tac and toc docstrings were incorrect.

fixes #314

[coveralls]

Coverage remained the same at 92.124% when pulling 27439cf on fix-timing-docstrings into a82ba69 on master.

[albop]

The docstring are OK. Looking at these functions again made me think one might sometime want to not print the elapsed time. Could be either toc(verbose=False) or (like in Julia) toq. Does it sound useful ?

[natashawatkins]

I agree - I think there could be some useful improvements to these functions such as suppressing output, adjusting units, choosing precision, etc.

[albop]

@natashawatkins : do you want to open an issue with the other improvements you can think of ? Then I'll close this PR as it is.

[natashawatkins]

No problem - done in #322

[albop]

OK

[albop]

@mmcky : I did the merge but can't close the PR. What did you actually expect me to do ?

[natashawatkins]

@albop the PR appears to be closed :)

[albop]

OK thanks !