STYLE: add more docs

Source/cmAddCustomCommandCommand.h

@@ -87,7 +87,10 @@ public:
       "ignored.\n"
       "The second signature adds a custom command to a target "
       "such as a library or executable. This is useful for "
-      "performing an operation before or after building the target:\n"
+      "performing an operation before or after building the target. "
+      "The command becomes part of the target and will only execute "
+      "when the target itself is built.  If the target is already built,"
+      " the command will not execute.\n"
       "  ADD_CUSTOM_COMMAND(TARGET target\n"
       "                     PRE_BUILD | PRE_LINK | POST_BUILD\n"
       "                     COMMAND command1 [ARGS] [args1...]\n"