- wrap several descriptions of functions in markdown code blocks to call them out as code
- wrap hex values in markdown code to allow them to standout more easily
- fix a few typos
- fix a few grammatical errors
- more consistently format markdown tables

Use the CFE_TIME_SysTime_t type more consistently across TBL where
time is being stored.  Avoid using seconds/subseconds directly.

This also adds an initializer value for CFE_TIME_SysTime_t that
has seconds/subseconds as zero.

Fix #2542, improve app dev guide

Fix #2543, consistent TIME values for TBL structures