CTest: Teach ctest_coverage() to read CTEST_COVERAGE_EXTRA_FLAGS

Use it to set the CoverageExtraFlags value just as COVERAGE_EXTRA_FLAGS
does in the CTest module for creating DartConfiguration.tcl.
Help: Add release notes for topic 'ctest-coverage-extra'
Merge topic 'ctest-coverage-extra' into next

93d5c4dc Help: Add release notes for topic 'ctest-coverage-extra'
ef1c1153 CTest: Teach ctest_coverage() to read CTEST_COVERAGE_EXTRA_FLAGS
Help: Document variables CMAKE_APPBUNDLE_PATH and CMAKE_FRAMEWORK_PATH
Merge topic 'doc-osx-path-variables' into next

764e977a Help: Document variables CMAKE_APPBUNDLE_PATH and CMAKE_FRAMEWORK_PATH