ENH: Added documentation for COMPILE_FLAGS property and clarified meaning of DEFINE_SYMBOL.

Source/cmSetTargetPropertiesCommand.h

@@ -85,7 +85,13 @@ public:
         "compiling sources in a shared library. "
         "If not set here then it is set to target_EXPORTS by default "
         "(with some substitutions if the target is not a valid C "
-        "identifier).\n"
+        "identifier). This is useful for headers to know whether they are "
+        "being included from inside their library our outside to properly "
+        "setup dllexport/dllimport decorations. "
+        "The COMPILE_FLAGS property sets additional compiler flags used "
+        "to build sources within the target.  It may also be used to pass "
+        "additional preprocessor definitions."
+        "\n"
         "For shared libraries VERSION and SOVERSION can be used to specify "
         "the build version and api version respectively. When building or "
         "installing appropriate symlinks are created if the platform "