Add comments explaining what lcov lines do.

tools/code_coverage_report.bash

@@ -30,11 +30,22 @@ for opt in "$@" ; do
 done
 
 mkdir -p $LCOVDIR
+
+# Generate initial zero-coverage data. This adds files that were otherwise not run to the report
 lcov -c --initial --rc lcov_branch_coverage=1 --directory build --output-file ${LCOVDIR}/initialcoverage.info
+
+# Capture executed code data.
 lcov -c --rc lcov_branch_coverage=1 --directory build --output-file ${LCOVDIR}/testcoverage.info
+
+# Combine the initial zero-coverage report with the executed lines report
 lcov -a ${LCOVDIR}/initialcoverage.info -a ${LCOVDIR}/testcoverage.info --rc lcov_branch_coverage=1 --o ${LCOVDIR}/fullcoverage.info
+
+# Only include files that are within this workspace (eg filter out stdio.h etc)
 lcov -e ${LCOVDIR}/fullcoverage.info "${PWD}/*" --rc lcov_branch_coverage=1 --output-file ${LCOVDIR}/workspacecoverage.info
+
+# Remove files in the build subdirectory because they are generated files (like messages, services, etc)
 lcov -r ${LCOVDIR}/workspacecoverage.info "${PWD}/build/*" --rc lcov_branch_coverage=1 --output-file ${LCOVDIR}/projectcoverage.info
+
 if [ $COVERAGE_REPORT = codecovio ]; then
  bash <(curl -s https://codecov.io/bash) -f ${LCOVDIR}/projectcoverage.info
 else