improve documentation on how we use gcov

Ticket #16792 (moved) (0.2.9.1-alpha) introduced some automation for excluding lines from filtered gcov output. We should document the prefixes it uses to mark excluded lines. There should also be information about how to read the gcov-diff files, which are processed to remove line numbers, among other things. We should also refer to the gcc documentation for gcov at https://gcc.gnu.org/onlinedocs/gcc/Invoking-Gcov.html