Add Brief Documentation Block to Test Property AWK Script

Mostly to aid future maintainers.

tests/getprop.awk

@@ -1,3 +1,28 @@
+# Extract test property values from CTestTestfile.cmake
+#
+# User must initialise two awk variables, typically through the "-v" option,
+# when invoking this script
+#
+#   search: Search pattern.  Typically a string such as
+#      set_tests_properties($test_name
+#
+#   prop: Property name, for instance DIRNAME or SIMULATOR.
+#
+# Property value will be printed to the standard output stream.
+#
+# The script assumes that $1 on candidate lines is suitable for matching
+# against 'search', and that $2 of the matching lines is the word
+# 'PROPERTIES'.
+#
+# Example:
+#   # Get value of SIMULATOR property in test named by shell variable
+#   # $failed_test and assign this to shell variable 'binary'.
+#
+#   binary=$(awk -v search="set_tests_properties\\\($failed_test" \
+#            -v prop="SIMULATOR" \
+#            -f getprop.awk \
+#            CTestTestfile.cmake)
+
 $1 ~ search {
     for (i = 3; i <= NF; ++i) {
         if ($i == prop) {