bpo-29026: Clarify documentation of time.time (#34)

* bpo-29026: Clarity documentation of time.time

Clarify the documentation of time.time by more
precisely defining what is meant by "seconds since
the epoch" on most platforms. Additionally explain
how gmtime and localtime may be used to extract
calendar components and convert to a more common
date format.

* bpo-29026: Minor improvements for time.time doc

* bpo-29026: Consistency fixes for time.time doc