ENH: Simplify docs args for define_property

This teaches the define_property command signature to accept multiple
arguments after the BRIEF_DOCS and FULL_DOCS keywords.  We append the
arguments together, making specification of long documentation easier.

Source/cmDefinePropertyCommand.cxx

@@ -98,13 +98,11 @@ bool cmDefinePropertyCommand
       }
     else if(doing == DoingBrief)
       {
-      doing = DoingNone;
-      this->BriefDocs = args[i];
+      this->BriefDocs += args[i];
       }
     else if(doing == DoingFull)
       {
-      doing = DoingNone;
-      this->FullDocs = args[i];
+      this->FullDocs += args[i];
       }
     else
       {

Source/cmDefinePropertyCommand.h

@@ -56,8 +56,8 @@ public:
         "  define_property(<GLOBAL | DIRECTORY | TARGET | SOURCE |\n"
         "                   TEST | VARIABLE | CACHED_VARIABLE>\n"
         "                   PROPERTY <name> [INHERITED]\n"
-        "                   BRIEF_DOCS <brief-doc>\n"
-        "                   FULL_DOCS <full-doc>)\n"
+        "                   BRIEF_DOCS <brief-doc> [docs...]\n"
+        "                   FULL_DOCS <full-doc> [docs...])\n"
         "Define one property in a scope for use with the "
         "set_property and get_property commands.  "
         "This is primarily useful to associate documentation with property "